Archive for the ‘Oracle XE’ Category
Create Oracle User
After you create and provision the Oracle Database 11g XE, you create an instance with the following two step process.
- Create a
studentOracle user account with the following command:CREATE USER student IDENTIFIED BY student DEFAULT TABLESPACE users QUOTA 200M ON users TEMPORARY TABLESPACE temp;
- Grant necessary privileges to the newly created
studentuser:GRANT CREATE CLUSTER, CREATE INDEXTYPE, CREATE OPERATOR , CREATE PROCEDURE, CREATE SEQUENCE, CREATE SESSION , CREATE TABLE, CREATE TRIGGER, CREATE TYPE , CREATE VIEW TO student;
As always, I hope this helps those looking for how to do something that’s less than clear because everybody uses tools.
Fedora 30 Missing Library
Having run into an obsolete library issue installing Oracle Database 18c XE on Fedora, Version 30, I opted to revert my student image to Oracle Database 11g XE. The installation went without issue but when I tried to log into SQL*Plus as the oracle user, I got the following error message:
sqlplus: error while loading shared libraries: libnsl.so.1: cannot open shared object file: No such file or directory |
The libnsl.so.1 library is no longer installed as part of the distribution for Fedora 28 forward but you can install it with the yum tool, like:
yum install -y libnsl |
Display detailed console log →
Last metadata expiration check: 1:35:44 ago on Sun 11 Aug 2019 07:28:07 PM MDT. Dependencies resolved. ============================================================================= Package Architecture Version Repository Size ============================================================================= Installing: libnsl x86_64 2.29-15.fc30 updates 97 k Transaction Summary ============================================================================= Install 1 Package Total download size: 97 k Installed size: 287 k Downloading Packages: libnsl-2.29-15.fc30.x86_64.rpm 134 kB/s | 97 kB 00:00 ----------------------------------------------------------------------------- Total 69 kB/s | 97 kB 00:01 Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction Preparing : 1/1 Installing : libnsl-2.29-15.fc30.x86_64 1/1 Running scriptlet: libnsl-2.29-15.fc30.x86_64 1/1 Verifying : libnsl-2.29-15.fc30.x86_64 1/1 Installed: libnsl-2.29-15.fc30.x86_64 Complete! |
If you attempted to run the oracle-xe utility to configure the database prior to adding this library, it fails to provision the instance without a message. You won’t get the message until you manually try to connect as the sysdba privileged user. At that point, you’ll determine the instance wasn’t provisioned.
You can see that the installation failed when the oracle-xe utility fails to print the following lines to the console after the options are entered:
Starting Oracle Net Listener...Done Configuring database...Done Starting Oracle Database 11g Express Edition instance...Done Installation completed successfully. |
After installing the missing library, the oracle-xe utility works correctly. Alas, it looks like I’ll never bother to sort the Oracle Database 18c XE issues because after this version of the image we are moving the courses to a PostgreSQL database. PostgreSQL offers the smaller footprint that supports the core learning objectives of the courses.
As always, I hope this helps those looking for a solution.
Python & Oracle 1
While Python is an interpreted language, Python is a very popular programming language. You may ask yourself why it is so popular? The consensus answers to why it’s so popular points to several factors. For example, Python is a robust high-level programming language that lets you:
- Get complex things done quickly
- Automate system and data integration tasks
- Solve complex analytical problems
You find Python developers throughout the enterprise. Development, release engineering, IT operations, and support teams all use Python to solve problems. Business intelligence and data scientists use Python because it’s easy to use.
Developers don’t generally write end-user applications in Python. They mostly use Python as scripting language. Python provides a simple syntax that lets developers get complex things done quickly. Python also provides you with a vast set of libraries that let you can leverage to solve problems. Those libraries often simplify how you analyze complex data or automate repetitive tasks.
This article explains how to use the Python programming language with the Oracle database. It shows you how to install and use the cx_Oracle library to query data. Part 2 will cover how you insert, update, and delete data in the Oracle database, and how you call and use PL/SQL stored functions and procedures.
The article has two parts:
- How you install and use
cx_Oraclewith the Oracle database - How you query data statically or dynamically
This audience for this article should know the basics of writing a Python program. If you’re completely new to Python, you may want to get a copy of Eric Matthes’ Python Crash Course: A Hands-On, Project-Based Introduction to Programming. More experienced developers with shell scripting backgrounds may prefer Al Sweigart’s Automate the Boring Stuff with Python.
This article uses Python 2.7, which appears to be the primary commercial version of Python in most organizations. At least, it’s what most vendors ship with Linux distros. It also happens to be the Python distro on Fedora Linux.
How you install and use cx_Oracle with the Oracle database
The first step requires that you test the current version of Python on your Operating System (OS). For the purpose of this paper, you use the student user account. The student user is in the sudoer list, which gives the account super user privileges.
You can find the Python version by opening a Terminal session and running the following command:
[student@localhost ~]$ python -V |
It displays:
Python 2.7.5 |
You can download the current version of the cx_Oracle library at the Python Software Foundation’s web site. At the time of writing, the current version of the cx_Oracle is the cx_Oracle 5.2.1 version. The cx_Oracle library is available for download as a Red Hat Package Manager (RPM) module.
You download the cx_Oracle-5.2.1-11g-py26-1.x86_64.rpm to the /tmp directory or to a sudoer-enabled user’s downloads directory. Let’s assume you download the RPM into the /tmp directory. After you download the RPM, you can install it with the yum utility with this syntax:
yum install -y /tmp/cx_Oracle-5.2.1-11g-py27-1.x86_64.rpm |
However, the most current version is now 7.0. You want the following file on Fedora 64-bit Linux, which can be found at the Python Package Index web site:
cx_Oracle-7.0.0-cp27-cp27mu-manylinux1_x86_64.whl |
A wheel file requires that you use the pip utility (make sure to upgrade to the current version), like:
sudo pip install cx_Oracle-7.0.0-cp27-cp27mu*.whl |
It should print the following to the console:
Processing ./cx_Oracle-7.0.0-cp27-cp27mu-manylinux1_x86_64.whl Installing collected packages: cx-Oracle Successfully installed cx-Oracle-7.0.0 |
The cx_Oracle library depends on the Oracle Client software, which may or may not be installed. It installs without a problem but would raise a runtime error when using the Python software. You can check whether cx_Oracle is installed with the following syntax:
rpm –qa oracle-instantclient11.2-basic |
If the oracle-instantclient11.2-basic library isn’t installed, the command returns nothing. If the oracle-instantclient11.2-basic library is installed it returns the following:
oracle-instantclient11.2-basic-11.2.0.4.0-1.x86_64 |
Assuming you don’t have the Oracle Client software installed, you should download it from Oracle’s Instant Client Downloads web page. After you download the RPM, you install the Oracle 11g Release 2 Client software with the following syntax:
yum install -y /tmp/oracle-instantclient11.2-basic-11.2.0.4.0-1.x86_64.rpm |
You now have the necessary software installed and configured to run and test Python programs that work with the Oracle database. Python uses a standard path configuration to look for Python modules or libraries. You can see that set of path values by connecting to the Python IDLE environment, which is the runtime environment. The IDLE environment is very much like the SQL*Plus environment.
You connect to the Python IDLE environment by typing the following:
python |
It opens the Python IDLE environment. It should display the following:
Python 2.7.5 (default, Apr 10 2015, 08:09:05) [GCC 4.8.3 20140911 (Red Hat 4.8.3-7)] on linux2 Type "help", "copyright", "credits" or "license" for more information. |
You import the sys library and then you can print the path elements with the following command:
>>> import sys print sys.path |
It should print the following for Python 2.7 in Fedora Linux:
['', '/usr/lib64/python27.zip', '/usr/lib64/python2.7', '/usr/lib64/python2.7/plat-linux2', '/usr/lib64/python2.7/lib-tk', '/usr/lib64/python2.7/lib-old', '/usr/lib64/python2.7/lib-dynload', '/usr/lib64/python2.7/site-packages', '/usr/lib64/python2.7/site-packages/gtk-2.0', '/usr/lib/python2.7/site-packages'] |
You can now test whether the environment works by typing the following commands in the IDLE environment:
>>> import cx_Oracle db = cx_Oracle.connect("student/student@xe") print db.version |
It prints:
11.2.0.2.0 |
The other two sections require you to test components inside Python files. That means you need to supplement the default Python path variable. You do that by adding values to the Python environment variable, which is $PYTHONPATH.
The following adds the /home/student/Code/python directory to the Python path variable:
export set PYTHONPATH=/home/student/Code/python |
Next, we create an connection.py file, which holds the following:
# Import the Oracle library. import cx_Oracle try: # Create a connection. db = cx_Oracle.connect("student/student@xe") # Print a message. print "Connected to the Oracle " + db.version + " database." except cx_Oracle.DatabaseError, e: error, = e.args print >> sys.stderr, "Oracle-Error-Code:", error.code print >> sys.stderr, "Oracle-Error-Message:", error.message finally # Close connection. db.close() |
The import statement adds the cx_Oracle library to the program scope. The cx_Oracle library’s connect function takes either the user name and password, or the user name, password, and TNS alias.
The except block differs from what you typically see. The code value maps to the SQLCODE value and the message value maps to the SQLERRM value.
You can test the connection.py file as follows in the /home/student/Code/python directory:
python connection.py |
It prints the following:
Connected to the Oracle 11.2.0.2.0 database. |
This section has shown you how to setup the cx_Oracle library, and how you can test the cx_Oracle library with Python programs.
How you query data statically or dynamically
The prior section shows you how to connect to an Oracle instance and how to verify the driver version of the cx_Oracle library. Like most ODBC and JDBC software, Python first creates a connection. Then, you need to create a cursor inside a connection.
The basicCursor.py program creates a connection and a cursor. The cursor holds a static SQL SELECT statement. The SELECT statement queries a string literal from the pseudo dual table.
# Import the Oracle library. import sys import cx_Oracle try: # Create a connection. db = cx_Oracle.connect("student/student@xe") # Create a cursor. cursor = db.cursor() # Execute a query. cursor.execute("SELECT 'Hello world!' FROM dual") # Read the contents of the cursor. for row in cursor: print (row[0]) except cx_Oracle.DatabaseError, e: error, = e.args print >> sys.stderr, "Oracle-Error-Code:", error.code print >> sys.stderr, "Oracle-Error-Message:", error.message finally: # Close cursor and connection. cursor.close() } db.close() |
The connect function assigns a database connection to the local db variable. The cursor function returns a cursor and assigns it to the local cursor variable. The execute function dispatches the query to Oracle’s SQL*Plus and returns the result set into a row element of the local cursor variable. The for-each loop reads the row element from the cursor variable and prints one row at a time. Since the cursor only returns a string literal, there’s only one row to return.
You test the program with this syntax:
python basicConnection.py |
It prints:
Hello world! |
The next basicTable.py program queries the item table. The item table holds a number of rows of data. The code returns each row inside a set of parentheses.
# Import the Oracle library. import cx_Oracle try: # Create a connection. db = cx_Oracle.connect("student/student@xe") # Create a cursor. cursor = db.cursor() # Execute a query. cursor.execute("SELECT item_title " + ", item_rating " + "FROM item " + "WHERE item_type = " " (SELECT common_lookup_id " + " FROM common_lookup " + " WHERE common_lookup_type = 'DVD_WIDE_SCREEN')") # Read the contents of the cursor. for row in cursor: print (row[0], row[1]) except cx_Oracle.DatabaseError, e: error, = e.args print >> sys.stderr, "Oracle-Error-Code:", error.code print >> sys.stderr, "Oracle-Error-Message:", error.message finally: # Close cursor and connection. cursor.close() db.close() |
The SQL query is split across several lines by using the + operator. The + operator concatenates strings, and it lets you format a long query statement. The range for loop returns tuples from the cursor. The tuples are determined by the SELECT-list of the query.
The query returns the following type of results:
('Casino Royale', 'PG-13') ... ('Star Wars - Episode I', 'PG') ('Star Wars - Episode II', 'PG') ('Star Wars - Episode III', 'PG-13') ('Star Wars - Episode IV', 'PG') ('Star Wars - Episode V', 'PG') ('Star Wars - Episode VI', 'PG') |
At this point, you know how to work with static queries. The next example shows you how to work with dynamic queries. The difference between a static and dynamic query is that an element of the string changes.
You have two options for creating dynamic strings. The first lets you glue a string inside a query. The second lets you embed one or more bind variables in a string. As a rule, you should use bind variables because they avoid SQL injection risks.
The following is the basicDynamicTable.py script
# Import the Oracle library. import cx_Oracle sRate = 'PG-13' try: # Create a connection. db = cx_Oracle.connect("student/student@xe") # Define a dynamic statment. stmt = "SELECT item_title, item_rating FROM item WHERE item_rating = :rating" # Create a cursor. cursor = db.cursor() # Execute a statement with a bind variable. cursor.execute(stmt, rating = sRate) # Read the contents of the cursor. for row in cursor: print (row[0], row[1]) except cx_Oracle.DatabaseError, e: error, = e.args print >> sys.stderr, "Oracle-Error-Code:", error.code print >> sys.stderr, "Oracle-Error-Message:", error.message finally: # Close cursor and connection. cursor.close() db.close() |
You need to assign a dynamic SQL statement to a local string variable. The bind variable is preceded with a colon (:). The execute function takes a string variable with the dynamic SQL statement. Then, you provide a name and value pair. The name needs to match the bind variable in the dynamic SQL statement. The value needs to map to a local Python variable.
The query should return a full list from the item table for the two item_title and item_rating columns:
('Casino Royale', 'PG-13')
...
('Harry Potter and the Goblet of Fire', 'PG-13')
('Harry Potter and the Order of the Phoenix', 'PG-13')
('The Lord of the Rings - Fellowship of the Ring', 'PG-13')
('The Lord of the Rings - Two Towers', 'PG-13')
('The Lord of the Rings - The Return of the King', 'PG-13')
('The Lord of the Rings - The Return of the King', 'PG-13') |
This article should have shown you how to effectively work static and dynamic queries. You can find the scripts on the github.com server.
Critical Triggers
Oracle Critical and Non-critical Triggers
This article demonstrates how you can write critical and non-critical row-level triggers. You may ask yourself, what are critical and non-critical triggers? That’s a great question. A critical trigger stops processing and raises an exception within the scope of an Application Programming Interface (API). An API is typically a series of end-user forms that help you solve business problems. A non-critical trigger either allows users to perform undesired behavior or it automatically fixes undesired behavior by preventing it. Non-critical triggers may log events but they don’t typically raise exceptions to the API.
Next, you’re probably asking yourself if critical and non-critical triggers are important. That’s also a great question. The answer is they’re very important and a key part of any database-centric application software solution.
If you’re new to database triggers, you can read the DML Trigger Basic article on this site to get an introduction. By way of review, you can write database triggers against DDL or DML statements. DML triggers can be either statement-level or row-level triggers.
The difference between a statement-level and row-level trigger is simple. A statement-level trigger runs once for any INSERT, UPDATE, or DELETE statement, which means you can’t inspect the specific rows that a DML statement affects. A row-level trigger runs once for each row affected by an INSERT, UPDATE, or DELETE statement.
Row-level database triggers give us the most granular (fancy word for detailed) view of transactions in your application. They’re also the best suited to logging changes happening with your data. The examples in this article will use DML row-level database triggers.
Business Logic
The article creates some tables for the examples, and the tables use traditional Oracle sequences and triggers. That’s because using sequences and triggers is the closest to how Oracle APEX creates tables. Many readers are familiar with how APEX works. After we create the tables, sequences, and basic automatic numbering database triggers, you will learn how to create non-critical triggers. The last section shows you how to create critical triggers.
It’s helpful to have a basic business problem when you work with so many moving parts. I chose a business problem that should be familiar to most people. The example uses a human resource professional. A human resource professional creates new employees when they join a company. Company policy sometimes dictates the convention for personal names. For example, they may restrict multipart last names. That means when you want to enter a multipart last name; they replace the whitespace with a hyphen.
The example business case requires that all last names must have hyphens. This means that the company disallows multipart last names. While this may seem old fashioned, it’s a simple business process to model, and it lets you see how to work with non-critical and critical database triggers.
So, here are our two use cases:
Non-critical Use Case
A human resource professional may try to enter a multipart last name with whitespace between parts. The entry may be intentional or simply a mistake. Assuming a positive mental attitude, you should assume the human resource profession doesn’t understand the policy. That means our triggers shouldn’t raise an exception when initially entering a value. The insert trigger should only log the attempt to enter non-conforming data. Initial entries, like this, are made through INSERT statements.
Critical Use Case
What the same human resource professional does when they notice that they weren’t able to enter a multipart last name becomes important. A critical trigger becomes necessary when the human resource professional tries to change a hyphenated name into a multipart name. The API uses an UPDATE statement to change an existing value with a new value. There is no use case when the human resource professional accepts the change to a hyphenated name.
The following steps you through how you create a framework for the non-critical and critical triggers. The framework uses three tables.
Framework
The non-critical trigger only uses two of those tables. The non-critical trigger is an INSERT trigger and the critical trigger is an UPDATE trigger. The application_user table will contain information about our authorized users; and the employee table will be the target for our non-critical and critical triggers.
The following creates the application_user table with this statement:
SQL> CREATE TABLE application_user 2 ( application_user_id NUMBER 3 , application_user_name VARCHAR2(30) CONSTRAINT application_user_nn1 NOT NULL 4 , created_by NUMBER CONSTRAINT application_user_nn2 NOT NULL 5 , creation_date DATE CONSTRAINT application_user_nn3 NOT NULL 6 , last_updated_by NUMBER CONSTRAINT application_user_nn4 NOT NULL 7 , last_update_date DATE CONSTRAINT application_user_nn5 NOT NULL 8 , CONSTRAINT application_user_pk PRIMARY KEY (application_user_id) 9 , CONSTRAINT application_user_fk1 FOREIGN KEY (created_by) 10 REFERENCES application_user (application_user_id) 11 , CONSTRAINT application_user_fk2 FOREIGN KEY (last_updated_by) 12 REFERENCES application_user (application_user_id)); |
The application_user_seq supports a surrogate key for the application_user table. You create it with the following statement:
SQL> CREATE OR REPLACE TRIGGER application_user_t1 2 BEFORE INSERT ON application_user 3 FOR EACH ROW 4 BEGIN 5 /* Check for a empty image_id primary key column value, 6 and assign the next sequence value when it is missing. */ 7 IF :NEW.application_user_id IS NULL THEN 8 SELECT application_user_seq.NEXTVAL 9 INTO :NEW.application_user_id 10 FROM dual; 11 END IF; 12 END; 13 / |
You will need at least one row in the application_user table to test the non-critical and critical triggers. The following insert a single row into the application_user table:
SQL> INSERT INTO application_user 2 ( application_user_name 3 , created_by 4 , creation_date 5 , last_updated_by 6 , last_update_date) 7 VALUES 8 ('Database Administrator' 9 , 1 10 , TRUNC(SYSDATE) 11 , 1 12 , TRUNC(SYSDATE)); |
The next statement creates the employee table:
SQL> CREATE TABLE employee 2 ( employee_id NUMBER 3 , employee_number VARCHAR2(10) 4 , first_name VARCHAR2(20) CONSTRAINT employee_nn1 NOT NULL 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20) CONSTRAINT employee_nn2 NOT NULL 7 , created_by NUMBER CONSTRAINT employee_nn3 NOT NULL 8 , creation_date DATE CONSTRAINT employee_nn5 NOT NULL 9 , last_updated_by NUMBER CONSTRAINT employee_nn6 NOT NULL 10 , last_update_date DATE CONSTRAINT employee_nn7 NOT NULL 11 , CONSTRAINT employee_pk PRIMARY KEY (employee_id) 12 , CONSTRAINT employee_fk1 FOREIGN KEY (created_by) 13 REFERENCES application_user (application_user_id) 14 , CONSTRAINT employee_fk2 FOREIGN KEY (last_updated_by) 15 REFERENCES application_user (application_user_id)); |
You create the employee_seq sequence with this statement:
SQL> CREATE SEQUENCE employee_seq; |
Next, you create a trigger to generate sequence values like you did for the application_user table:
SQL> CREATE OR REPLACE TRIGGER employee_t1 2 BEFORE INSERT ON employee 3 FOR EACH ROW 4 BEGIN 5 /* Check for a empty image_id primary key column value, 6 and assign the next sequence value when it is missing. */ 7 IF :NEW.employee_id IS NULL THEN 8 SELECT employee_seq.NEXTVAL 9 INTO :NEW.employee_id 10 FROM dual; 11 END IF; 12 END; 13 / |
You have created the two tables for our non-critical trigger. The next section relies on the framework and integrates with it.
Non-critical Trigger
Before you create the logging trigger, you should test the concept of replacing a whitespace in a multipart last name with a hyphenated name. The following INSERT trigger fixes user input by replacing the whitespace with a hyphen. It doesn’t log the entry and some times you won’t log results for this type of trigger.
You create the employee_t2 trigger with the following:
SQL> CREATE OR REPLACE TRIGGER employee_t2 2 BEFORE INSERT ON employee 3 FOR EACH ROW 4 FOLLOWS employee_t1 5 WHEN (REGEXP_LIKE(NEW.last_name,' ')) 6 BEGIN 7 /* Substitute a dash for the white space. */ 8 :NEW.last_name := REGEXP_REPLACE(:NEW.last_name,' ','-',1,1); 9 END; 10 / |
Line 4 designates that employee_t2 executes after employee_t1, which is the purpose of the FOLLOWS command. Line 8 uses the REGEXP_REPLACE function to find and replace the first instance of a whitespace with a hyphen.
After creating the employee_t2 trigger, you can test it by using an INSERT statement like this:
SQL> INSERT INTO employee 2 ( employee_number 3 , first_name 4 , last_name 5 , created_by 6 , creation_date 7 , last_updated_by 8 , last_update_date ) 9 VALUES 10 ('B12345-678' 11 ,'Sandy' 12 ,'Johnston Smith' 13 , 1 14 , TRUNC(SYSDATE) 15 , 1 16 , TRUNC(SYSDATE)); |
You can verify that the employee_t1 trigger prevented the entry of a multipart last name with the following query:
SQL> COLUMN employee_id FORMAT 9999 HEADING "Employee|ID #" SQL> COLUMN employee_number FORMAT A10 HEADING "Employee|Number" SQL> COLUMN first_name FORMAT A20 HEADING "First Name" SQL> COLUMN last_name FORMAT A20 HEADING "Last Name" SQL> SELECT employee_id 2 , employee_number 3 , first_name 4 , last_name 5 FROM employee; |
It returns:
Employee Employee
ID # Number First Name Last Name
-------- ---------- -------------------- --------------------
1 B12345-678 Sandy Johnston-Smith |
As you see from the results, the last name is hyphenated. If we accept another use case for the UPDATE statement, we may treat updates like you treat inserts.
An INSERT trigger doesn’t guarantee the user can’t change the hyphenated last name into a multipart last name. The application user can always change the value by using an UPDATE statement. That’s why there must be an UPDATE trigger.
The first element of a our
SQL> CREATE OR REPLACE TRIGGER employee_t3 2 BEFORE UPDATE OF last_name ON employee 3 FOR EACH ROW 4 WHEN (REGEXP_LIKE(NEW.last_name,' ')) 5 BEGIN 6 /* Substitute a dash for the white space. */ 7 :NEW.last_name := REGEXP_REPLACE(:NEW.last_name,' ','-',1,1); 8 END; 9 / |
Line 2 guarantees that the UPDATE trigger only runs when an UPDATE statement changes the last_name column of the employee table. An UPDATE statement like the following causes the trigger to run (technically, the jargon is “fire”):
SQL> UPDATE employee 2 SET last_name = 'Johnston Smith' 3 WHERE employee_number = 'B12345-678'; |
Having shown you how to create the non-critical INSERT and UPDATE triggers, I’ll now show you how to create the following employee_log table. This is where you can store the results from INSERT, UPDATE, and DELETE triggers. All columns are nullable (or optional) columns except the sequence generated employee_log_id column. The columns are optional because an INSERT statement never has an old set of values, and a DELETE statement never has a new set of values. Only the UPDATE statement provides old and new values inside a trigger.
The following creates the employee_log table:
SQL> CREATE TABLE employee_log 2 ( employee_log_id NUMBER 3 , employee_event VARCHAR2(6) 4 , old_employee_id NUMBER 5 , old_employee_number VARCHAR2(10) 6 , old_first_name VARCHAR2(20) 7 , old_middle_name VARCHAR2(20) 8 , old_last_name VARCHAR2(20) 9 , old_created_by NUMBER 10 , old_creation_date DATE 11 , old_last_updated_by NUMBER 12 , old_last_update_date DATE 13 , new_employee_id NUMBER 14 , new_employee_number VARCHAR2(10) 15 , new_first_name VARCHAR2(20) 16 , new_middle_name VARCHAR2(20) 17 , new_last_name VARCHAR2(20) 18 , new_created_by NUMBER 19 , new_creation_date DATE 20 , new_last_updated_by NUMBER 21 , new_last_update_date DATE 22 , CONSTRAINT employee_log_pk PRIMARY KEY (employee_log_id)); |
You should create the employee_log_seq sequence, like
SQL> CREATE SEQUENCE employee_log_seq; |
Then, you should add an employee_log_t1 trigger to generate the sequence value automatically. The trigger follows the pattern of the prior two triggers for the application_user and employee tables.
You create the employee_log_seq trigger with the following syntax:
SQL> CREATE OR REPLACE TRIGGER employee_log_t1 2 BEFORE INSERT ON employee_log 3 FOR EACH ROW 4 BEGIN 5 /* Check for a empty image_id primary key column value, 6 and assign the next sequence value when it is missing. */ 7 IF :NEW.employee_log_id IS NULL THEN 8 SELECT employee_log_seq.NEXTVAL 9 INTO :NEW.employee_log_id 10 FROM dual; 11 END IF; 12 END; 13 / |
The logging table is the first step. After creating the logging table, you need to create a standalone log_invalid_employee procedure. The following code creates the procedure. This procedure only runs in the current transaction context, and later another version shows you how to implement it in an autonomous transaction context.
SQL> CREATE OR REPLACE 2 PROCEDURE log_invalid_employee 3 ( pv_employee_event VARCHAR2 4 , pv_old_employee_id NUMBER 5 , pv_old_employee_number VARCHAR2 6 , pv_old_first_name VARCHAR2 7 , pv_old_last_name VARCHAR2 8 , pv_old_created_by NUMBER 9 , pv_old_creation_date DATE 10 , pv_old_last_updated_by NUMBER 11 , pv_old_last_update_date DATE 12 , pv_new_employee_id NUMBER 13 , pv_new_employee_number VARCHAR2 14 , pv_new_first_name VARCHAR2 15 , pv_new_last_name VARCHAR2 16 , pv_new_created_by NUMBER 17 , pv_new_creation_date DATE 18 , pv_new_last_updated_by NUMBER 19 , pv_new_last_update_date DATE) IS 20 BEGIN 21 /* Write to the log table. */ 22 INSERT INTO employee_log 23 ( employee_event 24 , old_employee_id 25 , old_employee_number 26 , old_first_name 27 , old_last_name 28 , old_created_by 29 , old_creation_date 30 , old_last_updated_by 31 , old_last_update_date 32 , new_employee_id 33 , new_employee_number 34 , new_first_name 35 , new_last_name 36 , new_created_by 37 , new_creation_date 38 , new_last_updated_by 39 , new_last_update_date ) 40 VALUES 41 ( pv_employee_event 42 , pv_old_employee_id 43 , pv_old_employee_number 44 , pv_old_first_name 45 , pv_old_last_name 46 , pv_old_created_by 47 , pv_old_creation_date 48 , pv_old_last_updated_by 49 , pv_old_last_update_date 50 , pv_new_employee_id 51 , pv_new_employee_number 52 , pv_new_first_name 53 , pv_new_last_name 54 , pv_new_created_by 55 , pv_new_creation_date 56 , pv_new_last_updated_by 57 , pv_new_last_update_date ); 58 END log_invalid_employee; 59 / |
With the logging table and procedure, you can now rework the INSERT and UPDATE triggers into a single trigger. The new trigger fires when an INSERT or an UPDATE statement affects the employee table. That means you can log the data from both events.
If you created employee_t1, employee_t2 and employee_t3 triggers, you need to drop employee_t2 and employee_t3 triggers before creating the new trigger. The previous employee_t3 trigger will cause incorrect behaviors because it is incompatible with the new employee_t1 trigger.
The new employee_t1 trigger is:
SQL> CREATE OR REPLACE TRIGGER employee_t1 2 BEFORE INSERT OR UPDATE OF last_name ON employee 3 FOR EACH ROW 4 WHEN (REGEXP_LIKE(NEW.last_name,' ')) 5 DECLARE 6 /* DML event label. */ 7 lv_employee_event VARCHAR2(6); 8 BEGIN 9 /* Check for an event and assign event value. */ 10 IF INSERTING THEN 11 /* Check for a empty image_id primary key column value, 12 and assign the next sequence value when it is missing. */ 13 IF :NEW.employee_id IS NULL THEN 14 SELECT employee_seq.NEXTVAL 15 INTO :NEW.employee_id 16 FROM dual; 17 END IF; 18 lv_employee_event := 'INSERT'; 19 ELSE 20 lv_employee_event := 'UPDATE'; 21 END IF; 22 23 /* Log the details captured by an insert or update. */ 24 log_invalid_employee 25 ( pv_employee_event => lv_employee_event 26 , pv_old_employee_id => :old.employee_id 27 , pv_old_employee_number => :old.employee_number 28 , pv_old_first_name => :old.first_name 29 , pv_old_last_name => :old.last_name 30 , pv_old_created_by => :old.created_by 31 , pv_old_creation_date => :old.creation_date 32 , pv_old_last_updated_by => :old.last_updated_by 33 , pv_old_last_update_date => :old.last_update_date 34 , pv_new_employee_id => :NEW.employee_id 35 , pv_new_employee_number => :NEW.employee_number 36 , pv_new_first_name => :NEW.first_name 37 , pv_new_last_name => :NEW.last_name 38 , pv_new_created_by => :NEW.created_by 39 , pv_new_creation_date => :NEW.creation_date 40 , pv_new_last_updated_by => :NEW.last_updated_by 41 , pv_new_last_update_date => :NEW.last_update_date ); 42 43 /* Substitute a dash for the white space. */ 44 :NEW.last_name := REGEXP_REPLACE(:NEW.last_name,' ','-',1,1); 45 END; 46 / |
This non-critical trigger checks whether the event is an INSERT statement on line 10. The trigger generates a sequence value when an INSERT statement fires the trigger. Then, the trigger sets a local variable with the INSERT string. It assigns an UPDATE string when an UPDATE statement fires the trigger.
After the event detection logic, the trigger calls the log_invalid_employee procedure on line 24. Line 44 changes the multipart last name into a hyphenated last name.
This part of the article has shown you how to create and manage non-critical triggers.
Critical Trigger
This part of the article shows you how to create and manage critical triggers. The key difference is that critical triggers stop the transaction that fires the trigger. This has significant impact on how you design and implement the log_invalid_employee procedure.
You need to modify the log_invalid_employee procedure so that it supports autonomous transactions. That requires adding a PRAGMA precompiler directive in the declaration block and a COMMIT statement after the INSERT statement.
The following shows you the changes required in the log_invalid_employee procedure:
SQL> CREATE OR REPLACE 2 PROCEDURE log_invalid_employee 3 ( pv_employee_event VARCHAR2 4 , pv_old_employee_id NUMBER ... 21 /* Set precompiler directive to run in a separate context. */ 22 PRAGMA AUTONOMOUS_TRANSACTION; 23 BEGIN ... 62 /* Commit the autonmous transaction. */ 63 COMMIT; 64 END log_invalid_employee; 65 / |
Line 22 holds the autonomous transaction PRAGMA, and line 63 holds the COMMIT statement. Both of these are required when you want to enable a trigger to both log data and raise an exception that terminates the transaction.
Next, you need to rework the employee_t1 trigger by adding content to the declaration and execution blocks, and by adding an exception block. The declaration block requires you to declare an exception variable and create a PRAGMA precompiler directive. The exception block requires you to add a conditional block at the end of the execution block. You also need to add an exception block to manage a raised exception.
The following shows you the changes required for the employee_t1 trigger:
SQL> CREATE OR REPLACE TRIGGER employee_t1 2 BEFORE INSERT OR UPDATE OF last_name ON employee 3 FOR EACH ROW 4 WHEN (REGEXP_LIKE(NEW.last_name,' ')) 5 DECLARE ... 9 /* Declare exception. */ 10 e EXCEPTION; 11 PRAGMA EXCEPTION_INIT(e,-20001); 12 BEGIN ... 47 /* Re-check for an event and assign event value. */ 48 IF INSERTING THEN 49 /* Substitute a dash for the white space. */ 50 :NEW.last_name := REGEXP_REPLACE(:NEW.last_name,' ','-',1,1); 51 ELSE 52 /* Throw exception. */ 53 RAISE_APPLICATION_ERROR(-20001,'No two-part last names without a hyphen.'); 54 END IF; 55 EXCEPTION 56 /* Capture an exception. */ 57 WHEN e THEN 58 ROLLBACK; 59 dbms_output.put_line('[Trigger Event: '||lv_employee_event||']'); 60 dbms_output.put_line(SQLERRM); 61 WHEN OTHERS THEN 62 dbms_output.put_line(SQLERRM); 63 END; 64 / |
Line 10 declares a local exception variable. Line 11 declares the PRAGMA precompiler directive. Lines 47 through 54 implements the conditional logic for writing a hyphenated last name for an INSERT statement, and the logic for raising an exception for an UPDATE statement.
An INSERT statement causes the database trigger to write to the employee_log logging table. An UPDATE statement causes the database trigger to write to the logging table and throw an exception.
The test case for a critical event trigger uses an UPDATE statement, as shown:
SQL> UPDATE employee 2 SET last_name = 'Zeta Jones' 3 WHERE employee_number = 'B98765-678'; |
The first thing you see is a thrown exception, like
[Trigger Event: UPDATE] ORA-20001: No two-part last names without a hyphen. |
After you see the thrown exception, you can run the following query to see what has been written to the exception_log table:
SQL> COLUMN employee_log_id FORMAT 9999 HEADING "Empl|Log|ID #" SQL> COLUMN old_employee_id FORMAT 9999 HEADING "Empl|ID #" SQL> COLUMN old_name FORMAT A25 HEADING "Old Name" SQL> COLUMN new_employee_id FORMAT 9999 HEADING "Empl|ID #" SQL> COLUMN new_name FORMAT A25 HEADING "New Name" SQL> SELECT employee_log_id 2 , old_employee_id 3 , DECODE( old_last_name || ', '|| old_first_name,', ',NULL 4 , old_last_name || ', '|| old_first_name) AS "old_name" 5 , new_employee_id 6 , DECODE( new_last_name || ', '|| new_first_name,', ',NULL 7 , new_last_name || ', '|| new_first_name) AS "new_name" 8 FROM employee_log; |
It displays:
Empl
Log Empl Empl
ID # ID # Old Name ID # New Name
----- ----- ------------------------- ----- -------------------------
1 2 Evert Lloyd, Chris
2 2 Evert-Lloyd, Chris 2 Evert Lloyd, Chris
3 3 Zeta Jones, Catherine
4 3 Zeta-Jones, Catherine 3 Zeta Jones, Catherine |
The ultimate test of these is that while there were many attempts at entering a multipart last name, none of them succeeds. You can query the last_name column from the employee table to verify that, like
SQL> SELECT last_name 2 FROM employee; |
It should show you the three rows that you’ve inserted and updated through this article. You should see:
Last Name ------------------------- Johnston-Smith Evert-Lloyd Zeta-Jones |
Through this article you should have learned how to create non-critical and critical triggers. These techniques are important when you manage transactions against business rules that can’t be supported by ordinary database constraints.
Oracle Trigger Basics
Oracle Trigger Basics
Once you master the basics of inserting, updating, and deleting data from the database, you typically learn about database triggers. Database triggers are coding elements that let you manage events beyond the limit of database constraints.
Before you can appreciate the power of database triggers, you need to understand what database constraints can and can’t do. Then, together we’ll explore how you can implement database triggers.
Database constraints let you manage events. A primary key constraint guarantees a column or a set of columns are unique and not null. A foreign key constraint guarantees a column only contains a value or set of values found in the primary key. A not null constraint makes a column mandatory when you insert or update a row in a table. A unique constraint guarantees a column or set of columns only exist in one row of a table. A check constraint guarantees a column’s value must comply with a set of rules defined with a row of data.
Database constraints do have limits. For example, a foreign key constraint doesn’t guarantee the right foreign key value because it only guarantees a foreign key value is a valid value in a list of possible values. That means it’s possible to insert or update a foreign key column or set of columns with an incorrect foreign key value. Only a database trigger can guarantee the insert or update of a correct foreign key value. The database trigger verifies the correct behavior by validating conditions before an insert or update.
While a unique constraint guarantees uniqueness and a check constraint guarantees compliance against a set of rules in a row, only a database trigger can guarantee the maximum number of like rows in a table that comply with a rule. Also, there is no constraint that manages inserts, updates, and deletes with dependencies on data in other tables.
A Data Manipulation Language (DML) trigger lets you manage these shortfalls and more. You have two options when implementing DML triggers. One implements a statement trigger and the other implements a row-level trigger. A statement-level trigger runs once for any and all rows affected by an INSERT, UPDATE, or DELETE statement. A row-level trigger runs once for each row affected by an INSERT, UPDATE, or DELETE statement.
Both of these triggers have two components – a trigger and a trigger body. The trigger defines what event to manage and the trigger body implements the logic that manages the event.
Statement-Level Triggers
You should create two tables to work with statement-level triggers. The first is the avenger table and the second is the avenger_log table. Your inserts, updates, and deletes to the avenger table act as events that fire triggers. Statement-level triggers can be defined to run before or after INSERT, UPDATE, and DELETE statements. Statement-level triggers are narrowly scoped events and they log message data to the avenger_log table.
This is the definition of the avenger table:
SQL> CREATE TABLE avenger 2 ( avenger_id NUMBER 3 , avenger_name VARCHAR2(30) 4 , first_name VARCHAR2(20) 5 , last_name VARCHAR2(20)); |
This is the definition of the avenger_log table:
SQL> CREATE TABLE avenger_log 2 ( avenger_log_id NUMBER 3 , trigger_name VARCHAR2(30) 4 , trigger_timing VARCHAR2(6) 5 , trigger_event VARCHAR2(6) 6 , trigger_type VARCHAR2(12)); |
The following avenger_t1 creates a BEFORE INSERT statement trigger:
SQL> CREATE OR REPLACE TRIGGER avenger_t1 2 BEFORE INSERT ON avenger 3 DECLARE 4 /* Declare local trigger-scope variables. */ 5 lv_sequence_id NUMBER := avenger_log_s.NEXTVAL; 6 lv_trigger_name VARCHAR2(30) := 'AVENGER_T1'; 7 lv_trigger_event VARCHAR2(6) := 'INSERT'; 8 lv_trigger_type VARCHAR2(12) := 'STATEMENT'; 9 lv_trigger_timing VARCHAR2(6) := 'BEFORE'; 10 BEGIN 11 /* Log event into the avenger_log table. */ 12 INSERT INTO avenger_log 13 ( avenger_log_id 14 , trigger_name 15 , trigger_event 16 , trigger_type 17 , trigger_timing ) 18 VALUES 19 ( lv_sequence_id 20 , lv_trigger_name 21 , lv_trigger_event 22 , lv_trigger_type 23 , lv_trigger_timing ); 24 END avenger_t1; 25 / |
Lines 1 and 2 declare the trigger. Lines 3 through 24 implements an anonymous PL/SQL block as the trigger’s body, and line 25 executes the trigger. Lines 6 through 9 store literal values for the trigger’s name, event, type, and timing. The trigger uses these literal values when logging events to the avenger_log table.
You access the data catalog information about triggers in the DBA_, ALL_, and USER_TRIGGERS views in a non-containerized database. Triggers also exist in those views for containerized databases (CDB). CDBs have an additional CDB_TRIGGERS view that stores triggers. The trigger body is stored in the TRIGGER_BODY column of those views in a LONG column.
You can create an AFTER STATEMENT trigger by simply changing the first two lines or the trigger declaration, as follows:
SQL> CREATE OR REPLACE TRIGGER avenger_t1 2 AFTER INSERT ON avenger |
Then, you need to change values of the string literals on lines 6, 7, and 9 as follows:
6 lv_trigger_name VARCHAR2(30) := 'AVENGER_T2'; 7 lv_trigger_event VARCHAR2(6) := 'INSERT'; 8 lv_trigger_type VARCHAR2(12) := 'STATEMENT'; 9 lv_trigger_timing VARCHAR2(6) := 'AFTER'; |
Compiling the database triggers, let’s insert a row into the avenger table, like this:
SQL> INSERT INTO avenger 2 VALUES 3 ( avenger_s.NEXTVAL 4 ,'Captain America' 5 ,'Steven' 6 ,'Rogers'); |
Then, you can query the avenger_log table, like this:
SQL> COLUMN avenger_log_id FORMAT 999 HEADING "Avenger|Log ID #" SQL> COLUMN trigger_name FORMAT A12 HEADING "Trigger|Name" SQL> COLUMN trigger_timing FORMAT A7 HEADING "Trigger|Timing" SQL> COLUMN trigger_event FORMAT A7 HEADING "Trigger|Event" SQL> COLUMN trigger_type FORMAT A12 HEADING "Trigger|Type" SQL> SELECT * FROM avenger_log; |
It returns two rows – one row from the avenger_t1 trigger and the other from the avenger_t2 trigger:
Avenger TRIGGER TRIGGER TRIGGER TRIGGER LOG ID # Name TIMING Event TYPE -------- ------------ ------- ------- ---------- 1 AVENGER_T2 AFTER INSERT STATEMENT 2 AVENGER_T1 BEFORE INSERT STATEMENT |
Both of the triggers use the avenger_log_s1 sequence. You may notice that the AFTER STATEMENT trigger ran before the BEFORE STATEMENT trigger. That shows you that triggers aren’t sequenced by default, even when you think that the timing event should sequence them.
Oracle lets you sequence triggers by using the FOLLOWS clause when you define database triggers. The following modifies the avenger_t2 by adding a FOLLOWS clause on line 3, and it uses ellipses to shorten the example:
SQL> CREATE OR REPLACE TRIGGER avenger_t2 2 BEFORE INSERT ON avenger 3 FOLLOWS avenger_t1 4 DECLARE … 11 BEGIN … 25 END avenger_t2; 26 / |
The testing script drops and creates the avenger_log table before creating fresh copies of the avenger_t1 and avenger_t2 triggers. The script lets you re-query the avenger_log table without the baggage of the previous two rows.
Like before, it returns two rows – one row from the avenger_t1 trigger and the other from the avenger_t2 trigger:
Avenger TRIGGER TRIGGER TRIGGER TRIGGER LOG ID # Name TIMING Event TYPE -------- ------------ ------- ------- ----------- 1 AVENGER_T1 BEFORE INSERT STATEMENT 2 AVENGER_T2 AFTER INSERT STATEMENT |
You should note that the BEFORE STATEMENT trigger now runs before the AFTER STATEMENT trigger. The FOLLOWS clause lets you guarantee the order of trigger execution.
As you can see, statement-level triggers don’t give us the ability to see, change, or log the before and after values of data. You can do that with row-level triggers.
Row-Level Triggers
Row-level triggers let you see the initial column values you add into a table with an INSERT statement. Row-level triggers let you see existing column values and the column values provided by an UPDATE statement. The DELETE statement only provides the existing column values to a trigger because it removes the row from the database. Inside the row-level trigger you can change new values based on rules that you put in code inside the database trigger.
The avenger_log table requires major changes to support a row-level database trigger because it needs to store the old and new values of a table’s data column. Data columns hold values that describe an instance of the table. A data column or set of data columns should also define a unique key that makes each row unique in a table.
After engineering a table, you should also add a surrogate key column. A surrogate (stand-in) key column contains a value generated from a sequence, and a surrogate key is generally unrelated to the subject of a table. You use the natural key to find a unique row in a table, and you copy the surrogate key value when you want a foreign key to link another row with the row identified by the surrogate key.
Both the surrogate key column and natural key (one or more columns) should both identify unique rows. That means for every surrogate key there should be a natural key.
The data columns in the avenger table are the avenger_name, first_name, and last_name columns. You should define an old and new column for each of the data columns when you create a logging table.
This defines the new avenger_log table:
SQL> CREATE TABLE avenger_log 2 ( avenger_log_id NUMBER 3 , trigger_name VARCHAR2(30) 4 , trigger_timing VARCHAR2(6) 5 , trigger_event VARCHAR2(6) 6 , trigger_type VARCHAR2(12) 7 , old_avenger_name VARCHAR2(20) 8 , old_first_name VARCHAR2(20) 9 , old_last_name VARCHAR2(20) 10 , new_avenger_name VARCHAR2(20) 11 , new_first_name VARCHAR2(20) 12 , new_last_name VARCHAR2(20)); |
The first row-level database trigger you create runs when an INSERT statement adds a new row to the avenger table. The code exists below:
SQL> CREATE OR REPLACE TRIGGER avenger_t3 2 BEFORE INSERT ON avenger 3 FOR EACH ROW 4 DECLARE 5 /* Declare local trigger-scope variables. */ 6 lv_sequence_id NUMBER := avenger_log_s.NEXTVAL; 7 lv_trigger_name VARCHAR2(30) := 'AVENGER_T3'; 8 lv_trigger_event VARCHAR2(6) := 'INSERT'; 9 lv_trigger_type VARCHAR2(12) := 'FOR EACH ROW'; 10 lv_trigger_timing VARCHAR2(6) := 'BEFORE'; 11 BEGIN 12 /* Log event into the avenger_log table. */ 13 INSERT INTO avenger_log 14 ( avenger_log_id 15 , trigger_name 16 , trigger_event 17 , trigger_type 18 , trigger_timing 19 , old_avenger_name 20 , old_first_name 21 , old_last_name 22 , new_avenger_name 23 , new_first_name 24 , new_last_name ) 25 VALUES 26 ( lv_sequence_id 27 , lv_trigger_name 28 , lv_trigger_event 29 , lv_trigger_type 30 , lv_trigger_timing 31 , :old.avenger_name 32 , :old.first_name 33 , :old.last_name 34 , :NEW.avenger_name 35 , :NEW.first_name 36 , :NEW.last_name ); 37 END avenger_t3; 38 / |
Line 3 declares the avenger_t3 trigger as a row-level trigger. Lines 31 through 36 inserts the old and new values from the row of the avenger table when the INSERT statement runs with the following three values:
SQL> INSERT INTO avenger 2 VALUES 3 ( avenger_s.NEXTVAL 4 ,'Capt. America' 5 ,'Steven' 6 ,'Rogers'); |
Since the script drops and recreates the avenger and avenger_log tables and drops the avenger_t1 and avenger_t2 statement-level triggers, you can write a query to return only the test row. The following anonymous PL/SQL block let’s you print the old and new column values next to one another. The program helps make the row-level trigger’s ability to see before and after values clear.
SQL> SET SERVEROUTPUT ON SIZE UNLIMITED SQL> BEGIN 2 FOR i IN (SELECT * FROM avenger_log) LOOP 3 dbms_output.put_line( 4 'Trigger Name [' 5 || i.trigger_name||']'); 6 dbms_output.put_line( 7 'Trigger Event [' 8 || i.trigger_event||']'); 9 dbms_output.put_line( 10 'Trigger Type [' 11 || i.trigger_type||']'); 12 dbms_output.put_line( 13 'Trigger Timing [' 14 || i.trigger_timing||']'); 15 dbms_output.put_line( 16 'Avenger Name [' 17 || i.old_avenger_name||'][' 18 || i.new_avenger_name||']'); 19 dbms_output.put_line( 20 'First Name [' 21 || i.old_first_name||'][' 22 || i.new_first_name||']'); 23 dbms_output.put_line( 24 'Last Name [' 25 || i.old_last_name||'][' 26 || i.new_last_name||']'); 27 END LOOP; 28 END; 29 / |
This anonymous block prints the following from the avenger_log table:
TRIGGER Name [AVENGER_T3] TRIGGER Event [INSERT] TRIGGER TYPE [FOR EACH ROW] TRIGGER TIMING [BEFORE] Avenger Name [][Capt. America] FIRST Name [][Steven] LAST Name [][Rogers] |
This has demonstrated how you write a row-level trigger against an INSERT event. You should note that the old values for the avenger_name, first_name, and last_name are null values between the square brackets. Next, you should examine how to write a row-level trigger against more than one type of event.
The Oracle Database lets you write individual triggers for INSERT, UPDATE, or DELETE statement, or a single trigger to manage INSERT, UPDATE, and DELETE events. The following modifies the avenger_t3 trigger so that it works for an INSERT, UPDATE, and DELETE events:
SQL> CREATE OR REPLACE TRIGGER avenger_t3 2 BEFORE INSERT OR UPDATE OR DELETE ON avenger 3 FOR EACH ROW |
Line 2 of the previous trigger is where we change the avenger_t3 trigger to also work with UPDATE and DELETE events. Then, we need to change one other line and then add a small IF-block to the trigger.
Line 8 of the original trigger assigns a default value to the lv_trigger_event variable, but you need to remove the value assignment. The modified line looks like this:
8 lv_trigger_event VARCHAR2(6); |
You also need to add an IF-block that manages Data Manipulation Language (DML) event functions. The IF-block should be the first thing in the execution block of the trigger body, and it should implement this logic:
11 BEGIN 12 /* Evaluate and assign event for logging. */ 13 IF INSERTING THEN lv_trigger_event := 'INSERT'; 14 ELSIF UPDATING THEN lv_trigger_event := 'UPDATE'; 15 ELSIF DELETING THEN lv_trigger_event := 'DELETE'; 16 END IF; ... |
The INSERTING event function on line 13 occurs when an INSERT statement activates the trigger. The UPDATING and DELETING event functions on lines 14 and 15 occur when a respective UPDATE or DELETE statement activity fires a trigger.
The following UPDATE statement now creates an event that the avenger_t3 trigger is monitoring:
SQL> UPDATE avenger 2 SET avenger_name = 'Captain America' 3 WHERE avenger_name = 'Capt. America'; |
Next, let’s test a DELETE statement with the following:
SQL> DELETE 2 FROM avenger 3 WHERE avenger_name = 'Captain America'; |
The following anonymous block program lets you see the log values inserted into the avenger_log table from the INSERT, UPDATE, and DELETE statement triggers:
SQL> SET SERVEROUTPUT ON SIZE UNLIMITED SQL> BEGIN 2 FOR i IN (SELECT * FROM avenger_log) LOOP 3 dbms_output.put_line( 4 'Row Number [' 5 || i.avenger_log_id ||'][' 6 || i.trigger_event ||']'); 7 dbms_output.put_line( 8 'Avenger Name [' 9 || i.old_avenger_name ||'][' 10 || i.new_avenger_name ||']'); 11 dbms_output.put_line( 12 'First Name [' 13 || i.old_first_name ||'][' 14 || i.new_first_name ||']'); 15 dbms_output.put_line( 16 'Last Name [' 17 || i.old_last_name ||'][' 18 || i.new_last_name ||']'); 19 END LOOP; 20 END; 21 / |
The anonymous block returns the following:
Row Number [1][INSERT] Avenger Name [][Capt. America] First Name [][Steven] Last Name [][Rogers] Row Number [2][UPDATE] Avenger Name [Capt. America][Captain America] First Name [Steven][Steven] Last Name [Rogers][Rogers] Row Number [3][DELETE] Avenger Name [Captain America][] First Name [Steven][] Last Name [Rogers][] |
You should notice the old values for the INSERT event are missing because there isn’t a row before running the INSERT statement. Likewise, you should notice the new values for the DELETE event are missing because there isn’t a row after running a DELETE statement. Only the UPDATE event has both an old and new value because the row exists before and after any change. The old values hold the row’s values before the UPDATE statement and the new values hold the row’s values after the UPDATE statement.
Column Substitutability
Object Types and Column Substitutability
This article shows you how to use extend parent (or superclass) objects. You extend parent classes when you implement specialized behaviors (or methods) in subtypes. That’s because SQL statements can’t work with specialized methods when a table’s column stores subclasses in a superclass column type.
Substitutability is the process of storing subtypes in a super type column. It is a powerful feature of the Oracle database. The “type evolution” feature of the Oracle Database 12c release makes it more important because it makes it more flexible. The flexibility occurs because Oracle lets you evolve parent classes.
You evolve parent classes when you implement MEMBER functions or procedures, and you want to access them for all substitutable column values. That’s necessary because you need to define the MEMBER function or procedure in the column’s base object type. Prior to Oracle Database 12c, you couldn’t change (evolve) a base type. If you’re new to the idea of object types and subtypes, you may want to check out my earlier “Object Types and Subtypes” article.
Before discussing the complexity of creating and evolving object types to support column substitutability, let’s create a base_t object type. The base_t object type will become our root node object type. A root node object type is our most general object type. A root node is also the topmost node of an inverted tree of object types. All subtypes of the root node become child nodes, and child nodes without their own children are at the bottom of the tree and they’re leaf nodes.
The following creates the base_t object type. It is similar to object types that I use in related articles to keep ideas consistent and simple across the articles. This version of the base_t object doesn’t try to maintain an internal unique identifier because the table maintains it as a surrogate key.
SQL> CREATE OR REPLACE 2 TYPE base_t IS OBJECT 3 ( oname VARCHAR2(30) 4 , CONSTRUCTOR FUNCTION base_t 5 RETURN SELF AS RESULT 6 , MEMBER FUNCTION get_oname RETURN VARCHAR2 7 , MEMBER PROCEDURE set_oname (oname VARCHAR2) 8 , MEMBER FUNCTION to_string RETURN VARCHAR2) 9 INSTANTIABLE NOT FINAL; 10 / |
The oname attribute on line two holds the name of the object type. Lines 4 and 5 define the default constructor, which has no formal parameters. Line 6 defines an accessor method, or getter, and line 7 defines a mutator, or setter. Line 8 defines a traditional to_string method that lets you print the contents of the object type.
Next, let’s implement the base_t object type’s body:
SQL> CREATE OR REPLACE 2 TYPE BODY base_t IS 3 /* A default constructor w/o formal parameters. */ 4 CONSTRUCTOR FUNCTION base_t 5 RETURN SELF AS RESULT IS 6 BEGIN 7 self.oname := 'BASE_T'; 8 RETURN; 9 END; 10 /* An accessor, or getter, method. */ 11 MEMBER FUNCTION get_oname RETURN VARCHAR2 IS 12 BEGIN 13 RETURN self.oname; 14 END get_oname; 15 /* A mutator, or setter, method. */ 16 MEMBER PROCEDURE set_oname 17 ( oname VARCHAR2 ) IS 18 BEGIN 19 self.oname := oname; 20 END set_oname; 21 /* A to_string conversion method. */ 22 MEMBER FUNCTION to_string RETURN VARCHAR2 IS 23 BEGIN 24 RETURN '['||self.oname||']'; 25 END to_string; 26 END; 27 / |
Line 7 assigns a literal value to the oname attribute. Line 24 returns the value of the oname attribute for the instance. The remainder of the object type is generic. You can read about the generic features in my “Object Types and Bodies Basics” and about accessor and mutator methods in my “Object Types with Getters and Setters” articles.
Let’s define and implement a hobbit_t subtype of our base_t object type. The hobbit_t object type is:
SQL> CREATE OR REPLACE TYPE hobbit_t UNDER base_t 2 ( genus VARCHAR2(20) 3 , name VARCHAR2(20) 4 , CONSTRUCTOR FUNCTION hobbit_t 5 ( genus VARCHAR2 6 , name VARCHAR2) RETURN SELF AS RESULT 7 , MEMBER FUNCTION get_genus RETURN VARCHAR2 8 , MEMBER FUNCTION get_name RETURN VARCHAR2 9 , MEMBER PROCEDURE set_genus (genus VARCHAR2) 10 , MEMBER PROCEDURE set_name (name VARCHAR2) 11 , OVERRIDING MEMBER FUNCTION to_string RETURN VARCHAR2) 12 INSTANTIABLE NOT FINAL; 13 / |
Lines 2 and 3 add two new genus and name attributes to the hobbit_t subtype. The hobbit_t subtype also inherits the oname attribute from its parent base_t type. Lines 7 and 8 define two getters and lines 9 and 10 define two setters, which support the genus and name attributes of the hobbit_t subtype. The hobbit_t object type’s getter and setters are unique to the subtype. They are also a specialization of the base_t object type. As such, these getters and setters are inaccessible to instances of the base_t object type. Line 11 defines an overriding to_string function for the base_t type’s to_string function.
The following implements the hobbit_t object body:
SQL> CREATE OR REPLACE TYPE BODY hobbit_t IS 2 /* A default constructor with two formal parameters. */ 3 CONSTRUCTOR FUNCTION hobbit_t 4 ( genus VARCHAR2 5 , name VARCHAR2 ) 6 RETURN SELF AS RESULT IS 7 BEGIN 8 self.oname := 'HOBBIT_T'; 9 self.name := name; 10 self.genus := genus; 11 RETURN; 12 END; 13 /* An accessor, or getter, method. */ 14 MEMBER FUNCTION get_genus RETURN VARCHAR2 IS 15 BEGIN 16 RETURN self.genus; 17 END get_genus; 18 /* An accessor, or getter, method. */ 19 MEMBER FUNCTION get_name RETURN VARCHAR2 IS 20 BEGIN 21 RETURN self.name; 22 END get_name; 23 /* A mutator, or setter, method. */ 24 MEMBER PROCEDURE set_genus 25 ( genus VARCHAR2 ) IS 26 BEGIN 27 self.genus := genus; 28 END set_genus; 29 /* A mutator, or setter, method. */ 30 MEMBER PROCEDURE set_name 31 ( name VARCHAR2 ) IS 32 BEGIN 33 self.name := name; 34 END set_name; 35 /* A to_string conversion method. */ 36 OVERRIDING MEMBER FUNCTION to_string RETURN VARCHAR2 IS 37 BEGIN 38 /* Uses general invocation on parent to_string 39 function. */ 40 RETURN (self AS base_t).to_string 41 || '['||self.genus||']['||self.name||']'; 42 END to_string; 43 END; 44 / |
Lines 4 and 5 list the parameters for the hobbit_t constructor. Line 8 assigns a literal value to the oname attribute of the base_t object type. Lines 9 and 10 assign the formal parameters to the genus and name attributes of the hobbit_t subtype. Line 40 uses a general invocation statement to call the base_t’s to_string function.
You can now create a table that has a substitutable column that uses the base_t parent object type. The Oracle database assumes object type columns are substitutable at all levels, unless you turn off a column’s substitutability.
The following creates a tolkien table, and it has only two columns. One column has a NUMBER data type and the other has a user-defined object type. The base_t object type column is substitutable at all levels:
SQL> CREATE TABLE tolkien 2 ( tolkien_id NUMBER 3 , character BASE_T ); |
You create a tolkien_s sequence for the unique tolkien_id column with the following:
SQL> CREATE SEQUENCE tolkien_s START WITH 1001; |
You can insert one base_t and two hobbit_t object types with the following INSERT statements:
SQL> INSERT INTO tolkien VALUES 2 ( tolkien_s.NEXTVAL, base_t() ); SQL> INSERT INTO tolkien VALUES 2 ( tolkien_s.NEXTVAL, hobbit_t('HOBBIT','Bilbo') ); SQL> INSERT INTO tolkien VALUES 2 ( tolkien_s.NEXTVAL, hobbit_t('HOBBIT','Frodo') ); |
The following simple query shows you the unique identifier in the tolkien_id column and collapsed object types in the character column of the tolkien table:
SQL> COLUMN character FORMAT A40 SQL> SELECT tolkien_id 2 , character 3 FROM tolkien; |
It should display the following:
TOLKIEN_ID CHARACTER(ONAME)
---------- ----------------------------------------
1001 BASE_T('BASE_T')
1002 HOBBIT_T('HOBBIT_T', 'HOBBIT', 'Bilbo')
1003 HOBBIT_T('HOBBIT_T', 'HOBBIT', 'Frodo') |
Oracle always stores object instances as collapsed object instances in tables. You need to use the TREAT function in SQL to read instances of an object type.
The TREAT function lets you place in memory an instance of an object type. The TREAT function requires that you designate the type of object instance. If you want the TREAT function to work with all rows of the table, you designate the column’s object type as the base (or superclass) type. Designating a subtype to work like a parent, grandparent, or any antecedent type is a form of casting. Though casting in this case is actually dynamic dispatch.
Dynamic dispatch lets you pass any subtype as a parent or antecedent type. Dynamic dispatch inspects the object and treats it as a unique object.
The following query uses the TREAT function to read the parent and any subtype of the parent object type:
SQL> COLUMN to_string FORMAT A40 SQL> SELECT tolkien_id 2 , TREAT(character AS BASE_T).to_string() AS to_string 3 FROM tolkien; |
It prints the oname attribute for base_t instances and the oname, genus, and name attributes for hobbit_t instances, like
TOLKIEN_ID TO_STRING ---------- --------------------------- 1001 [BASE_T] 1002 [BASE_T] 1003 [HOBBIT_T][HOBBIT][Bilbo] 1004 [HOBBIT_T][HOBBIT][Frodo] |
The TREAT function manages dynamic dispatch but requires any specialized method of a subtype to exist in the parent or antecedent type to which it is cast. Any query can cast to the root or an intermediate parent subtype. The TREAT function raises an exception when you don’t have an equivalent method stub (definition) in the parent or antecedent type.
For example, let’s modify the previous query and change the method call on line 2 from the to_string function to the get_name function. The new query is:
SQL> COLUMN to_string FORMAT A40 SQL> SELECT tolkien_id 2 , TREAT(character AS BASE_T).get_name() AS get_name 3 FROM tolkien; |
It fails with the following error:
, TREAT(character AS BASE_T).get_name() AS get_name * ERROR at line 2: ORA-00904: "STUDENT"."BASE_T"."GET_NAME": invalid identifier |
The reason for the failure is interesting. It occurs because the get_name function is only part of the hobbit_t subtype and can’t be found as an identifier inside the base_t object type. PL/SQL identifiers are: reserved or key words; predefined identifiers; quoted identifiers; user-identifiers; and user-defined variables, subroutine, and data or object type names.
You can access the MEMBER functions or procedures (method) of a subtype when you cast to a parent type provided you meet two conditions. First, you must implement the MEMBER method in the subtype. Second, you must define the same MEMBER method in the parent type.
Accessing a subtype MEMBER method differs from general invocation. General invocation occurs when you call a MEMBER method from a parent or antecedent type from a subtype’s OVERRIDING MEMBER method. Oracle doesn’t explain how you call a subtype’s method from a parent or antecedent type but there is a close corollary – packages.
For example, you can only call a package function or procedure from another PL/SQL block when you’ve defined it in the package specification. This means you need to implement a stub for the get_name function inside the base_t object type because it acts as the specification.
You add a get_name function to the base_t object type in the next example:
SQL> CREATE OR REPLACE 2 TYPE base_t IS OBJECT 3 ( oname VARCHAR2(30) 4 , CONSTRUCTOR FUNCTION base_t 5 RETURN SELF AS RESULT 6 , MEMBER FUNCTION get_name RETURN VARCHAR2 7 , MEMBER FUNCTION get_oname RETURN VARCHAR2 8 , MEMBER PROCEDURE set_oname (oname VARCHAR2) 9 , MEMBER FUNCTION to_string RETURN VARCHAR2) 10 INSTANTIABLE NOT FINAL; 11 / |
Line 6 adds the get_name function to the base_t object type. The following shows you how to implement get_name function stub in the object type body:
SQL> CREATE OR REPLACE 2 TYPE BODY base_t IS 3 CONSTRUCTOR FUNCTION base_t 4 RETURN SELF AS RESULT IS 5 BEGIN 6 self.oname := 'BASE_T'; 7 RETURN; 8 END; 9 MEMBER FUNCTION get_name RETURN VARCHAR2 IS 10 BEGIN 11 RETURN NULL; 12 END get_name; 13 MEMBER FUNCTION get_oname RETURN VARCHAR2 IS 14 BEGIN 15 RETURN self.oname; 16 END get_oname; 17 MEMBER PROCEDURE set_oname 18 ( oname VARCHAR2 ) IS 19 BEGIN 20 self.oname := oname; 21 END set_oname; 22 MEMBER FUNCTION to_string RETURN VARCHAR2 IS 23 BEGIN 24 RETURN '['||self.oname||']'; 25 END to_string; 26 END; 27 / |
Lines 9 through 12 implement the get_name function stub. You should note that it returns a null value because the name attribute doesn’t exist in the root node (base_t) object type.
The change to the hobbit_t object type is simpler. All you need to do is add the OVERRIDING keyword before the get_name member function in the hobbit_t object type and body. With that change, you can successfully run the following query:
SQL> COLUMN get_name FORMAT A20 SQL> SELECT tolkien_id 2 , TREAT(character AS BASE_T).get_name() AS get_name 3 FROM tolkien; |
It now works and prints:
TOLKIEN_ID GET_NAME ---------- -------------------- 1001 1002 Bilbo 1003 Frodo |
This article showed you how to extend parent object types. It also showed you how to modify parent types to support generalized calls with the TREAT function. Together these principles show you how to leverage substitutability on columns.
Types & Subtypes
Object Types and Subtypes
This article teaches you how to use subtypes or subclasses. You can define an object type with or without dependencies. Object types can have two types of dependencies. The simplest case occurs when you define an object attribute with an object type instead of a data type. The more complex case occurs when you define an object subtype because it inherits the behavior of the base object type. The base object type is a superclass and a parent class. The subtype is a subclass and a child class.
The ability to capture various result sets is a key use case for object types and subtypes. That’s because you can define a table’s column with the object type, and then you can store in that column the object type or any of its subtypes.
A base object type should contain a unique identifier and an object name. The Object Types & Bodies Basic article explains the best practice for unique identifiers. It suggests that you populate the unique ID value with a no argument constructor function. The object name attribute should hold the object type name.
I’d like to suggest we consider base_t as the name of our superclass. You can define a base_t object type like this:
SQL> CREATE OR REPLACE 2 TYPE base_t IS OBJECT 3 ( obj_id NUMBER 4 , obj_name VARCHAR2(30) 5 , CONSTRUCTOR FUNCTION base_t RETURN SELF AS RESULT 6 , MEMBER FUNCTION to_string RETURN VARCHAR2) 7 INSTANTIABLE NOT FINAL; 8 / |
Line 2 and 3 define two attributes. They are the unique identifier, or ID, and the object. The no argument constructor function assigns values to the obj_id and obj_name attributes. It assigns the base_t_s sequence value to the obj_id attribute and it assigns a string literal to the obj_name attribute. The to_string member function returns a concatenated string of the obj_id and obj_name values. The return value of the to_string function is what you want to disclose about the contents of an object type.
Line 7 declares the class as instantiable and not final. You can create an instance of a class when its instantiable, and you can create subtypes of a type when it’s NOT FINAL.
You need to create the base_t_s sequence before we can compile the base_t object body. The following statement creates the base_t_s sequence as a set of values starting at 1:
SQL> CREATE SEQUENCE base_t_s; |
The object body for the base_t object type is:
SQL> CREATE OR REPLACE 2 TYPE BODY base_t IS 3 4 /* Default constructor. */ 5 CONSTRUCTOR FUNCTION base_t RETURN SELF AS RESULT IS 6 BEGIN 7 /* Assign a sequence value and string literal 8 to the instance. */ 9 self.obj_id := base_t_s.NEXTVAL; 10 self.obj_name := 'BASE_T'; 11 RETURN; 12 END; 13 14 /* A text output function. */ 15 MEMBER FUNCTION to_string RETURN VARCHAR2 IS 16 BEGIN 17 RETURN 'UID#: ['||obj_id||']'||CHR(10) 18 || 'Type: ['||obj_name||']'; 19 END; 20 / |
Line 9 assigns a base_t_s sequence value to the obj_id attribute, which serves as a unique identifier. Line 10 assigns a string literal to the obj_name attribute. The obj_name attribute identifies the object type. Line 17 and 18 prints the contents of the base_t object type as a two-row string.
You can test the construction of the base_t object type with this query:
SQL> SELECT base_t() FROM dual; |
It displays:
BASE_T()(OBJ_ID, OBJ_NAME) ---------------------------- BASE_T(1, 'BASE_T') |
Alternatively, you can test the to_string member function with the TREAT function, like:
SQL> SELECT TREAT(base_t() AS base_t).to_string() AS "Text" 2 FROM dual; |
It displays:
Text ---------------- UID#: [2] Type: [BASE_T] |
Alternatively, you can test to_string member function with an anonymous block (by enabling SERVEROUTPUT):
SQL> SET SERVEROUTPUT ON SIZE UNLIMITED SQL> BEGIN 2 dbms_output.put_line(base_t().to_string); 3 END; 4 / |
It displays:
Text ---------------- UID#: [2] Type: [BASE_T] |
There’s another way to query the object instance with a query. While I don’t think it’s effective for this situation, you should know how the syntax works. It requires that you create a collection of the base_t object type, which you can do with this syntax:
SQL> CREATE OR REPLACE 2 TYPE base_t_tab IS TABLE OF base_t; 3 / |
It displays:
Text ---------------- UID#: [2] Type: [BASE_T] |
You can query the base_t object type from inside a collection by using the CAST and COLLECT functions. The COLLECT function puts a single object instance into a base_t_tab collection. The CAST function puts the generic collection into a specific collection.
The syntax to perform this operation is:
SQL> COLUMN obj_id FORMAT 9999 SQL> COLUMN obj_name FORMAT A20 SQL> SELECT * 2 FROM TABLE(SELECT CAST(COLLECT(base_t()) AS base_t_tab) 3 FROM dual); |
Assuming the base_t_s sequence holds a current value of 3, the query returns:
OBJ_ID OBJ_NAME ------ -------------------- 5 BASE_T |
This type of query isn’t too useful in day-to-day programming. It’s more of a corner use case for testing an object type with a sequence value. While you expect an obj_id value of 4, the query returns a value of 5. Somewhere in the execution Oracle appears to call the sequence twice.
The COLLECT and TREAT functions increment the value of sequence when you put them inside object types. So, you shouldn’t use a sequence as a unique identifier inside an object type. I plan to cover the better approach in subsequent article.
Now that you have a solid base_t object, let’s create a hobbit_t subtype. The hobbit_t subtype adds one attribute to the two attributes in the base_t object type.
The following declares the hobbit_t object type as a subtype and overrides the to_string member function:
SQL> CREATE OR REPLACE 2 TYPE hobbit_t UNDER base_t 3 ( hobbit_name VARCHAR2(30) 4 , CONSTRUCTOR FUNCTION hobbit_t 5 ( hobbit_name VARCHAR2 ) RETURN SELF AS RESULT 6 , OVERRIDING MEMBER FUNCTION to_string RETURN VARCHAR2) 7 INSTANTIABLE NOT FINAL; 8 / |
Assuming the base_t_s sequence holds a current value of 3, the query returns:
OBJ_ID OBJ_NAME ------ -------------------- 5 BASE_T |
Line 2 declares the hobbit_t subtype as UNDER the base_t object type. There isn’t a no argument constructor that mirrors the parent base_t object type. You also can’t call the parent type’s constructor like they do in Java.
Line 4 and 5 declare a single argument constructor. The hobbit_t object type’s constructor assigns values to the obj_id and obj_name attributes. More or less it performs the same function as its parent’s constructor. Then, the constructor assigns the parameter value to the hobbit_name attribute of the hobbit_t object type.
Line 6 declares an overriding to_string member function. The overriding to_string member function replaces the behavior of our parent class. It provides the subclass with its own a specialized behavior.
You implement the hobbit_t object type like this:
SQL> CREATE OR REPLACE 2 TYPE BODY hobbit_t IS 3 4 /* One argument constructor. */ 5 CONSTRUCTOR FUNCTION hobbit_t 6 ( hobbit_name VARCHAR2 ) RETURN SELF AS RESULT IS 7 BEGIN 8 /* Assign a sequence value and string literal 9 to the instance. */ 10 self.obj_id := base_t_s.NEXTVAL; 11 self.obj_name := 'HOBBIT_T'; 12 13 /* Assign a parameter to the subtype only attribute. */ 14 self.hobbit_name := hobbit_name; 15 RETURN; 16 END; 17 18 /* An output function. */ 19 OVERRIDING MEMBER FUNCTION to_string RETURN VARCHAR2 IS 20 BEGIN 21 RETURN (self AS base_t).to_string||CHR(10) 22 || 'Name: ['||hobbit_name||']'; 23 END; 24 END; 25 / |
Lines 10 assigns a sequence value to the obj_id attribute. Line 11 assigns a string literal to the obj_name attribute. Line 14 assigns the parameter value of the constructor to the hobbit_name attribute of the hobbit_t subtype. Line 21 is more complex than a simple assignment.
Line 21 contains a “generalized invocation” of the base_t object. A generalized invocation calls a parent or super class method. PL/SQL member functions or procedures are methods. Line 21 calls the base_t type’s to_string function. This way, the overriding to_string function returns a specialized result. It returns the result from the parent class and the value of its own hobbit_name attribute.
You can test the generalized invocation with the following query:
SQL> SELECT 2 TREAT( 3 hobbit_t('Bilbo') AS hobbit_t).to_string() AS "Text" 4 FROM dual; |
The query prints:
Text ----------------------- UID#: [1] Type: [HOBBIT_T] Name: [Bilbo] |
Together we’ve explored of how you create types and subtypes. You’ve learned a type is a generalization or superclass, and a subtype is a specialization or subclass. You’ve also learned how to create both a generalization and specialization. At this point, you may ask, “Why should I bother with subtypes?”
The benefit of subtypes is dynamic dispatch. Dynamic dispatch is the process of selecting an object type from an inverted tree of object types. The topmost object type is the root node or most generalized version of an object type. The bottom most object type is a leaf node or the most specialized version of an object type. All nodes between the root node and leaf nodes are simply nodes. Nodes become more specialized as you step down the hierarchy from the root node.
The process of selecting an object type from an inverted tree is polymorphism. Polymorphism means your program specifies the most general node at compile time. Then, the program accepts the root node or any subordinate nodes at runtime. Moreover, dynamic dispatch is like writing a function or procedure to do many things.
Another form of dynamic dispatch occurs when you overload a function or procedure in a PL/SQL package. Calls to overloaded functions or procedure choose which version to run based on the data types of the call parameters.
The key difference between overloading and selecting object types is simple. The first deals with choosing between different data types or object types. The second deals with choosing between object types in the same node tree.
You have two choices to demonstrate dynamic dispatch. One would use a SQL table or varray collection and the other would use column substitutability. Creating a table that uses substitutability seems the easiest approach.
The following creates a table of the base_t object type:
SQL> CREATE TABLE dynamic 2 ( character_type BASE_T ); |
You can now insert a base_t object type or any of the base_t subtypes. The base_t_s sequence is reset for the test case INSERT statements:
SQL> INSERT INTO dynamic VALUES (base_t()); SQL> INSERT INTO dynamic VALUES (hobbit_t('Bilbo Baggins')); SQL> INSERT INTO dynamic VALUES (hobbit_t('Peregrin Took')); |
The following query uses a CASE statement to identify whether the column returns a base_t or hobbit_t object type:
SQL> SELECT 2 CASE 3 WHEN TREAT(character_type AS hobbit_t) IS NOT NULL THEN 4 TREAT(character_type AS hobbit_t).to_string() 5 ELSE 6 TREAT(character_type AS base_t).to_string() 7 END AS "Text" 8 FROM dynamic; |
The query returns the following:
Text ----------------------- UID#: [3] Type: [BASE_T] UID#: [7] Type: [HOBBIT_T] Name: [Bilbo Baggins] UID#: [13] Type: [HOBBIT_T] Name: [Peregrin Took] |
The result set shows you that the character_type column holds different types of the base_t object type. It should also show you how you may store different result logs from DML row level triggers in a single table. Another article, I hope to write soon.
The unique identifier appears to increment three times with the first INSERT statement and five times with subsequent inserts. Actually, each INSERT statement increments the sequence five times. A debug statement would show you that it assigns the third call to the .NEXTVAL pseudo column value to the obj_id value. This is true for both the base_t and hobbit_t object type, and any other derived subtypes.
This article has shown you how to implement object types and subtypes. It also has explained how dynamic dispatch works and it provides a working example of dynamic dispatch leveraging column substitutability.
Type Getters & Setters
Object Types with Getters and Setters
This article is for you when you know the basics about how you work Oracle’s object types. It teaches you how to write effective getters, setters, comparators, and static methods. Please read my Object Types & Bodies Basic article if you’re not sure how to work with object types.
Getters access an object instance and return values from an instance variable. Along with getters, you have setters. Setters let you assign a new value to an instance variable. Formally, getters are accessor methods and setters are mutator methods. PL/SQL implements getters as functions and setters as procedures. After all a PL/SQL procedure is like a function that returns a void data type in Java.
The Object Types & Bodies Basic article introduces a people_obj object type. This article extends the behavior of the people_obj type. Extends is a funny word because it can have different meanings in object-oriented programming. Here, extends means to add functionality.
The first things we’ll add are getters and setters for all the attributes of the object instance. We need to add them to the object type and body because Oracle implements objects like it does packages. The object type defines the published functions and procedures. The object body implements the published functions and procedures.
Here’s the new people_obj type with getters and setters:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER 4 , first_name VARCHAR2(20) 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20) 7 , CONSTRUCTOR FUNCTION people_obj RETURN SELF AS RESULT 8 , CONSTRUCTOR FUNCTION people_obj 9 ( first_name VARCHAR2 10 , middle_name VARCHAR2 DEFAULT NULL 11 , last_name VARCHAR2 ) RETURN SELF AS RESULT 12 , MEMBER FUNCTION get_people_id RETURN NUMBER 13 , MEMBER FUNCTION get_first_name RETURN VARCHAR2 14 , MEMBER FUNCTION get_middle_name RETURN VARCHAR2 15 , MEMBER FUNCTION get_last_name RETURN VARCHAR2 16 , MEMBER PROCEDURE set_first_name (pv_first_name VARCHAR2) 17 , MEMBER PROCEDURE set_middle_name (pv_first_name VARCHAR2) 18 , MEMBER PROCEDURE set_last_name (pv_first_name VARCHAR2)) 19 INSTANTIABLE NOT FINAL; 20 / |
The new getters and setters are on lines 12 through 18. The closing parenthesis for the list of attributes, functions, and procedures moves from line 11 to line 18. While there are four attributes in the people_obj type and four getters for those attributes, there are only three setters. The reason for the difference is simple. The people_id attribute is a unique identifier. You should never change the value of a unique identifier.
Next, lets implement the object body. I’m opting to show the complete object body because some readers may not check out the earlier article. Here’s the people_obj body:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS 3 4 /* Default constructor. */ 5 CONSTRUCTOR FUNCTION people_obj RETURN SELF AS RESULT IS 6 7 /* Set a counter variable using a sequence. */ 8 lv_people_obj_s NUMBER := people_obj_s.NEXTVAL; 9 10 BEGIN 11 /* Assign a sequence value to the instance. */ 12 self.people_id := lv_people_obj_s; 13 14 /* Return a constructed instance. */ 15 RETURN; 16 END people_obj; 17 18 /* Override constructor. */ 19 CONSTRUCTOR FUNCTION people_obj 20 ( first_name VARCHAR2 21 , middle_name VARCHAR2 DEFAULT NULL 22 , last_name VARCHAR2 ) RETURN SELF AS RESULT IS 23 24 /* Create a empty default instance. */ 25 people PEOPLE_OBJ := people_obj(); 26 27 BEGIN 28 /* Create the instance with the default constructor. */ 29 people.first_name := first_name; 30 people.middle_name := middle_name; 31 people.last_name := last_name; 32 33 /* Assign a local instance this instance. */ 34 self := people; 35 36 /* Return the current instance. */ 37 RETURN; 38 END people_obj; 39 40 /* Get people ID attribute. */ 41 MEMBER FUNCTION get_people_id RETURN NUMBER IS 42 BEGIN 43 RETURN self.people_id; 44 END get_people_id; 45 46 /* Get first name attribute. */ 47 MEMBER FUNCTION get_first_name RETURN VARCHAR2 IS 48 BEGIN 49 RETURN self.first_name; 50 END get_first_name; 51 52 /* Get middle name attribute. */ 53 MEMBER FUNCTION get_middle_name RETURN VARCHAR2 IS 54 BEGIN 55 RETURN self.middle_name; 56 END get_middle_name; 57 58 /* Get last name attribute. */ 59 MEMBER FUNCTION get_last_name RETURN VARCHAR2 IS 60 BEGIN 61 RETURN self.last_name; 62 END get_last_name; 63 64 /* Set first name attribute. */ 65 MEMBER PROCEDURE set_first_name 66 ( pv_first_name VARCHAR2 ) IS 67 BEGIN 68 self.first_name := pv_first_name; 69 END set_first_name; 70 71 /* Set middle name attribute. */ 72 MEMBER PROCEDURE set_middle_name 73 ( pv_middle_name VARCHAR2 ) IS 74 BEGIN 75 self.middle_name := pv_middle_name; 76 END set_middle_name; 77 78 /* Set last name attribute. */ 79 MEMBER PROCEDURE set_last_name 80 ( pv_last_name VARCHAR2 ) IS 81 BEGIN 82 self.last_name := pv_last_name; 83 END set_last_name; 84 END; 85 / |
The get_people_id member function on lines 41-44 returns the unique identifier for the object instance. The get_first_name member function on lines 47-50 returns the first_name attribute. The get_middle_name member function on lines 53-56 returns the middle_name attribute. The get_last_name member function on lines 59-62 returns the last_name attribute. Each of these getters returns an instance attribute. The self reserved word identifies the current instance of the object type.
The set_first_name member procedure on lines 65-69 assigns a value to the first_name attribute. The set_middle_name procedure on lines 72-76 assigns a value to the middle_name attribute. The set_last_name member procedures on lines 79-83 assigns a value to the last_name attribute. The constructor functions create instances of the people_obj and return them to the calling scope. Each of these setters assigns a value to an instance attribute.
Comparative functions are limited to the MAP and ORDER member functions. The MAP function only works with the CHAR, DATE, NUMBER, or VARCHAR2 data type. You could implement a MAP function against the last_name attribute but not the collection of the three variable length strings. You would implement an ORDER member function to compare the collection of strings.
You can define an equals MAP function in the people_obj object type like:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER ... 19 , MAP MEMBER FUNCTION equals RETURN VARCHAR2) 20 INSTANTIABLE NOT FINAL; 21 / |
After creating the people_obj object type, you can implement the following MAP function:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS ... 85 /* Implement an equals MAP function. */ 86 MAP MEMBER FUNCTION equals RETURN VARCHAR2 IS 87 BEGIN 88 RETURN self.last_name; 89 END equals; 90 91 END; 92 / |
The MAP function is inadequate when you compare multiple attributes. You can implement an ORDER MEMBER function with the following syntax in the people_obj object type.
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER ... 19 , ORDER MEMBER FUNCTION equals 20 (pv_people PEOPLE_OBJ) RETURN NUMBER) 21 INSTANTIABLE NOT FINAL; 22 / |
The ORDER function is more complete than the MAP function. You can implement a last name, first name, and middle name ORDER function as follows:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS ... 85 /* Implement an equals MAP function. */ 86 ORDER MEMBER FUNCTION equals 87 (pv_people PEOPLE_OBJ) RETURN NUMBER IS 88 BEGIN 89 IF NVL(self.last_name,'A') > NVL(pv_people.last_name,'A') THEN 90 RETURN 1; 91 ELSIF NVL(self.last_name,'A') = NVL(pv_people.last_name,'A') AND 92 NVL(self.first_name,'A') > NVL(pv_people.first_name,'A') THEN 93 RETURN 1; 94 ELSIF NVL(self.last_name,'A') = NVL(pv_people.last_name,'A') AND 95 NVL(self.first_name,'A') = NVL(pv_people.first_name,'A') AND 96 NVL(self.middle_name,'A') > NVL(pv_people.middle_name,'A') THEN 97 RETURN 1; 98 ELSE 99 RETURN 0; 100 END IF; 101 END equals; 102 END; 103 / |
The equals ORDER function on lines 86 through 101 checks for a three conditions. First, it checks whether the instance’s last_name is greater than the parameter object’s last_name. Second, it checks whether the last names are equal and the instance’s first_name is greater than the parameter object’s first_name. Finally, it checks whether the last and first names are equal and the middle_name is greater than the parameter object’s middle_name value.
Unfortunately, it’s hard to test this comparison without adding a to_string function. The to_string function prints the formatted name. You can add the to_string function to the object type like so:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER ... 19 , MAP MEMBER FUNCTION equals RETURN VARCHAR2 21 , MEMBER FUNCTION to_string RETURN VARCHAR2) 20 INSTANTIABLE NOT FINAL; 21 / |
Line 21 shows the declaration of the to_string function, and the following code snippet shows you the implementation of the to_string function:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS ... 103 /* Create a to_string function. */ 104 MEMBER FUNCTION to_string RETURN VARCHAR2 IS 105 BEGIN 106 RETURN self.last_name || ', ' || self.first_name || ' ' || 107 self.middle_name; 108 END to_string; 109 110 END; 111 / |
After assembling all the parts, we can test whether the ORDER comparative function works. The following anonymous block program declares a people_list collection that holds instances of the people_obj object type.
SQL> DECLARE 2 /* Declare an object type. */ 3 TYPE people_list IS TABLE OF people_obj; 4 5 /* Declare three object types. */ 6 lv_obj1 PEOPLE_OBJ := people_obj('Fred',NULL,'Maher'); 7 lv_obj2 PEOPLE_OBJ := people_obj('John',NULL,'Fedele'); 8 lv_obj3 PEOPLE_OBJ := people_obj('James',NULL,'Fedele'); 9 lv_obj4 PEOPLE_OBJ := people_obj('James','Xavier','Fedele'); 10 11 /* Declare a list of the object type. */ 12 lv_objs PEOPLE_LIST := people_list( lv_obj1, lv_obj2 13 , lv_obj3, lv_obj4); 14 15 /* Swap A and B. */ 16 PROCEDURE swap 17 ( a IN OUT PEOPLE_OBJ 18 , b IN OUT PEOPLE_OBJ ) IS 19 /* Declare a third variable. */ 20 c PEOPLE_OBJ; 21 BEGIN 22 /* Swap values. */ 23 c := b; 24 b := a; 25 a := c; 26 END swap; 27 28 BEGIN 29 /* Nested loop comparison. */ 30 FOR i IN 1..lv_objs.COUNT LOOP 31 FOR j IN 1..lv_objs.COUNT LOOP 32 IF lv_objs(i).equals(lv_objs(j)) = 0 THEN 33 swap(lv_objs(i), lv_objs(j)); 34 END IF; 35 END LOOP; 36 END LOOP; 37 38 /* Print the reordered list. */ 39 FOR i IN 1..lv_objs.COUNT LOOP 40 dbms_output.put_line(lv_objs(i).to_string()); 41 END LOOP; 42 END; 43 / |
The people_obj instances on lines 6 through 9 are out of order in the starting collection. The local swap procedure reorders them on lines 30 through 36. You would see the following output from the preceding anonymous block:
Fedele, James Fedele, James Xavier Fedele, John Maher, Fred |
All of our work in this paper so far shows you how to work with implementing functions and procedures in instances of object types. PL/SQL object types support MEMBER functions and procedures to work with object instances. PL/SQL object types also support STATIC functions and procedures. You use STATIC functions and procedures when you want to write and call a module in an object type that works like a function or procedure in a package.
You can call a STATIC function or procedure without creating an instance of an object. Creating an instance of the object type is a key use of STATIC functions. This approach is very much like how Oracle implements temporary BLOB and CLOB columns.
Here’s the snippet of additional code required in the people_obj object type:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER ... 21 , MEMBER FUNCTION to_string RETURN VARCHAR2 22 , STATIC FUNCTION get_people_obj 23 ( pv_people_id NUMBER) RETURN people_obj) 20 INSTANTIABLE NOT FINAL; 21 / |
The get_people_obj function is a STATIC function and it takes a single number to return a name. It accomplishes this by using a parameterized cursor. You would implement the get_people_obj function like so:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS ... 109 /* Create a get_people_obj function. */ 110 STATIC FUNCTION get_people_obj 111 ( pv_people_id NUMBER ) RETURN PEOPLE_OBJ IS 112 113 /* Implement a cursor. */ 114 CURSOR get_people_obj 115 ( cv_people_id NUMBER ) IS 116 SELECT first_name 117 , middle_name 118 , last_name 119 FROM contact 120 WHERE contact_id = cv_people_id; 121 122 /* Create a cursor variable. */ 123 lv_contact get_people_obj%ROWTYPE; 124 125 /* Create a temporary instance of people_obj. */ 126 lv_people_obj PEOPLE_OBJ; 127 BEGIN 128 /* Open, fetch and close cursor. */ 129 OPEN get_people_obj(pv_people_id); 130 FETCH get_people_obj INTO lv_contact; 131 lv_people_obj := people_obj( first_name => lv_contact.first_name 132 , middle_name => lv_contact.middle_name 133 , last_name => lv_contact.last_name); 134 CLOSE get_people_obj; 135 RETURN lv_people_obj; 136 END get_people_obj; 137 138 END; 139 / |
The get_people_obj function takes a single numeric parameter. The numeric parameter passes the primary key value for the contact table. Then, the STATIC function returns an instance of the people_obj object type. It accomplishes that feat by using the numeric value as a lookup key in the contact table, as you can see in the get_people_obj cursor on lines 114 through 120. The STATIC method opens, fetches a single row, and closes on lines 129 through 135.
Now you can call the get_people_obj function in a query and return an instance of people_obj. You can also use the to_string method to view the output, as follows:
SQL> SELECT people_obj.get_people_obj(1003).to_string() 2 FROM dual; |
It prints:
PEOPLE_OBJ.GET_PEOPLE_OBJ(1003).TO_STRING() --------------------------------------------- Vizquel, Oscar |
This article has shown you how to write effective getters, setters, comparators, and static methods. It also has shown how to test and work with Oracle object types and bodies.
Type & Body Basics
Object Types and Bodies Basics
Oracle Database 10g gave us a new way to write PL/SQL – object types. Object types are different from standard PL/SQL functions, procedures, and packages. While you can pin packages in memory, object types go one step further. You can instantiate them, which means you can start them, assign values to their variables, and put them into your PGA’s memory. Object types provide you with new challenges writing programs in the Oracle database.
Oracle Database 12c makes using object types simpler. That’s because Oracle Database 12c supports type evolution. Type evolution lets you change an object type when it has dependents. An object type’s dependents can be a table, another object type, function, procedure, or package. Oracle Database 12c also lets you white list the callers of an object type.
You define object types with variables and methods, like you define packages. Object type methods are either functions or procedures. You can implement object type functions and procedures as instance or static methods. An instance method works on the object type’s variable, whereas, static methods work like ordinary functions and procedures. That means static methods can’t access object type variables.
You learn how to define and implement basic object types and bodies in this article. This article shows you how to use and deploy objects and shows you how to implement the specialized CONSTRUCTOR functions.
The following declares a basic people_obj object type:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER 4 , first_name VARCHAR2(20) 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20)); 7 / |
The CREATE OR REPLACE is SQL syntax creates an object type, like you would create a PL/SQL function, procedure, or package. Lines 2 through 6 declare a four element people_obj object type, and the semicolon on line 6 acts as a statement terminator. The forward slash on line 7 executes the CREATE TYPE statement.
To most developers the foregoing syntax appears to declare a record data structure. There’s more to it than that. The CREATE TYPE syntax also creates an implicit constructor function. You can call the people_obj constructor with a list of parameter that matches both the list of element names and their data types. The call syntax supports both named and positional notation.
You can test the people_obj object type with the following anonymous block:
SQL> DECLARE 2 people PEOPLE_OBJ := people_obj(1,'John','Paul','Jones'); 3 BEGIN 4 dbms_output.put_line( people.first_name || ' ' 5 ||people.middle_name || ' ' 6 ||people.last_name); 7 END; 8 / |
Line 2 declares a variable of the object type with positional notation, and then it assigns an instance of the people_obj object type. On the right side of the assignment operator, a call to the constructor function creates an instance of the people_obj object type. Object construction has the highest order of precedence, which means it always creates the people_obj instance first.
Lines 4 through 6 print the values of the first, middle, and last name elements. These values are the instance values held by the peoplevariable. It prints:
John Paul Jones |
The following example shows you how to call the default people_obj constructor with named notation:
SQL> DECLARE 2 people PEOPLE_OBJ := people_obj( first_name => 'John' 3 , middle_name => 'Paul' 4 , last_name => 'Jones' 5 , people_id => 2); 6 BEGIN ... 10 END; 11 / |
The named notation on lines 2 through 5 let us vary the order of the object attributes. Oracle raises the following exception if you pare the list of call parameters by removing one of them.
PLS-00306: wrong number or types of arguments in call to 'PEOPLE_OBJ' |
You can add one or more override constructor functions to the people_obj object type. The first override constructor example has two call parameters, and they are the first_name and last_name parameters.
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER 4 , first_name VARCHAR2(20) 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20) 7 , CONSTRUCTOR FUNCTION people_obj 8 ( first_name VARCHAR2 9 , last_name VARCHAR2 ) RETURN SELF AS RESULT) 10 INSTANTIABLE NOT FINAL; 11 / |
Lines 7 through 9 declare the override constructor function. This override constructor function doesn’t provide a value for the people_id attribute. The concept of an object having a unique identifier, or ID, is part of good object-oriented design practices.
An Oracle sequence can help us guarantee the unique ID. You can create a people_obj_s sequence for the people_obj with the following syntax:
SQL> CREATE SEQUENCE people_obj_s; |
You can use the people_obj_s sequence in the override constructor to generate the unique ID. The following code implements the modified people_obj object type:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS 3 CONSTRUCTOR FUNCTION people_obj 4 ( first_name VARCHAR2 5 , last_name VARCHAR2 ) RETURN SELF AS RESULT IS 6 7 /* Set a counter variable using a sequence. */ 8 lv_people_obj_s NUMBER := people_obj_s.NEXTVAL; 9 10 BEGIN 11 /* Create the instance with the default constructor. */ 12 self := people_obj( people_id => lv_people_obj_s 13 , first_name => first_name 14 , middle_name => NULL 15 , last_name => last_name ); 16 /* Return the current instance. */ 17 RETURN; 18 END people_obj; 19 END; 20 / |
Line 8 declares a local lv_people_obj_s variable, and it assigns the next value from the people_obj_s sequence. The local variable is necessary because you can’t put a call to the .NEXTVAL pseudo column inside a call to an object type constructor function.
The self key word on line 12 represents the instance of an object. You call the default constructor on lines 12 through 15. The default constructor takes a local variable, two parameter values, and a null value.
You can test the new people_obj with the following anonymous block:
SQL> DECLARE 2 people PEOPLE_OBJ := people_obj( first_name => 'John' 3 , last_name => 'Jones'); 4 BEGIN 5 dbms_output.put_line( '['|| people.people_id ||'] ' 6 ||'['|| people.first_name ||'] ' 7 ||'['|| people.middle_name ||'] ' 8 ||'['|| people.last_name ||']'); 9 END; 10 / |
It prints
[1] [John] [] [Jones] |
Clearly, the handling of the middle_name attribute is suboptimal. Actually, it’s more or less a joke. However, it does give us an opportunity to show how to handle optional parameters in a constructor function.
You would change the people_obj object type by adding a parameter to the override constructor function, like
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER 4 , first_name VARCHAR2(20) 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20) 7 , CONSTRUCTOR FUNCTION people_obj 8 ( first_name VARCHAR2 9 , middle_name VARCHAR2 DEFAULT NULL 10 , last_name VARCHAR2 ) RETURN SELF AS RESULT) 11 INSTANTIABLE NOT FINAL; 12 / |
There are only two changes to the implementation of the people_obj object body. One changes the list of parameters in the constructor function. The other replaces the null assignment with a parameter value from the overriding constructor function.
Here’s the implementation of the new people_obj object body:
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS 3 CONSTRUCTOR FUNCTION people_obj 4 ( first_name VARCHAR2 5 , middle_name VARCHAR2 DEFAULT NULL 6 , last_name VARCHAR2 ) RETURN SELF AS RESULT IS 7 8 /* Set a counter variable using a sequence. */ 9 lv_people_obj_s NUMBER := people_obj_s.NEXTVAL; 10 11 BEGIN 12 /* Create the instance with the default constructor. */ 13 self := people_obj( people_id => lv_people_obj_s 14 , first_name => first_name 15 , middle_name => middle_name 16 , last_name => last_name ); 17 /* Return the current instance. */ 18 RETURN; 19 END people_obj; 20 END; 21 / |
Line 5 specifies the middle_name parameter as an optional parameter. The optional parameter in the middle of the list can present a problem when you make call to it with positional notation. A call with named notation on the other hand works without a hitch. Line 15 replaces the null value with the middle_name parameter from the constructor function.
You can test the modified people_obj with the following anonymous block:
SQL> DECLARE 2 people PEOPLE_OBJ := people_obj( first_name => 'John' 3 , last_name => 'Jones'); 4 5 BEGIN 6 dbms_output.put_line( '['|| people.people_id ||'] ' 7 ||'['|| people.first_name ||'] ' 8 ||'['|| people.middle_name ||'] ' 9 ||'['|| people.last_name ||']'); 10 END; 11 / |
It prints
[1] [John] [] [Jones] |
If you modify the constructor call on lines 2 through 4, as follows:
2 people PEOPLE_OBJ := people_obj( first_name => 'James' 3 , middle_name => 'Wilson' 4 , last_name => 'Jones'); |
It prints
[1] [John] [Wilson] [Jones] |
There are still several problems with the current people_obj object type. The largest shortfall is that there’s no traditional default constructor. In many object-oriented language, a default constructor is a null argument constructor. A null argument constructor let’s you position logic that all other constructors can leverage.
A sequence value is an example of logic that you can share across constructor functions. The following version of the people_obj object type declares a standard no argument constructor function:
SQL> CREATE OR REPLACE 2 TYPE people_obj IS OBJECT 3 ( people_id NUMBER 4 , first_name VARCHAR2(20) 5 , middle_name VARCHAR2(20) 6 , last_name VARCHAR2(20) 7 , CONSTRUCTOR FUNCTION people_obj RETURN SELF AS RESULT 8 , CONSTRUCTOR FUNCTION people_obj 9 ( first_name VARCHAR2 10 , middle_name VARCHAR2 DEFAULT NULL 11 , last_name VARCHAR2 ) RETURN SELF AS RESULT) 12 INSTANTIABLE NOT FINAL; 13 / |
Line 7 holds the declaration of a no argument constructor. The following people_obj object type implements a no argument constructor. The object body also makes access to the sequence a feature available to all overriding constructors.
SQL> CREATE OR REPLACE 2 TYPE BODY people_obj IS 3 4 /* Default constructor. */ 5 CONSTRUCTOR FUNCTION people_obj RETURN SELF AS RESULT IS 6 7 /* Set a counter variable using a sequence. */ 8 lv_people_obj_s NUMBER := people_obj_s.NEXTVAL; 9 10 BEGIN 11 /* Assign a sequence value to the instance. */ 12 self.people_id := lv_people_obj_s; 13 14 /* Return a constructed instance. */ 15 RETURN; 16 END; 17 18 /* Override constructor. */ 19 CONSTRUCTOR FUNCTION people_obj 20 ( first_name VARCHAR2 21 , middle_name VARCHAR2 DEFAULT NULL 22 , last_name VARCHAR2 ) RETURN SELF AS RESULT IS 23 24 /* Create a empty default instance. */ 25 people PEOPLE_OBJ := people_obj(); 26 27 BEGIN 28 /* Create the instance with the default constructor. */ 29 people.first_name := first_name; 30 people.middle_name := middle_name; 31 people.last_name := last_name; 32 33 /* Assign a local instance this instance. */ 34 self := people; 35 36 /* Return the current instance. */ 37 RETURN; 38 END people_obj; 39 END; 40 / |
The implementation of the no argument constructor is on lines 5 through 16. It uses the .NEXTVAL pseudo column to secure the next sequence value as a unique ID. Then, the constructor function returns a uniquely identified but otherwise empty object instance.
Line 25 creates a people_obj instance inside the declaration block of the overriding constructor. Inside the execution block, the overriding parameters are assigned to the attributes of the local instance. Ultimately, the local instance is assigned to the current instance and returned to any caller of the overriding constructor.
You call the modified overriding function with the following anonymous block:
SQL> DECLARE 2 people PEOPLE_OBJ := people_obj( first_name => 'Samuel' 3 , middle_name => 'Langhorne' 4 , last_name => 'Clemens'); 5 BEGIN 6 dbms_output.put_line( '['|| people.people_id ||'] ' 7 ||'['|| people.first_name ||'] ' 8 ||'['|| people.middle_name ||'] ' 9 ||'['|| people.last_name ||']'); 10 END; 11 / |
It prints
[3] [Samuel] [Langhorne] [Clemens] |
This article has shown you how to define and implement basic object types and bodies. It also has shown you how to work with default, no argument, and overriding constructor functions.
Type Dependency Tree
While trying to explain a student question about Oracle object types, it seemed necessary to show how to write a dependency tree. I did some poking around and found there wasn’t a convenient script at hand. So, I decided to write one.
This assumes the following Oracle object types, which don’t have any formal methods (methods are always provided by PL/SQL or Java language implementations):
CREATE OR REPLACE TYPE base_t AS OBJECT ( base_id NUMBER ) NOT FINAL; / CREATE OR REPLACE TYPE person_t UNDER base_t ( first_name VARCHAR2(20) , middle_name VARCHAR2(20) , last_name VARCHAR2(20)) NOT FINAL; / CREATE OR REPLACE TYPE driver_t UNDER person_t ( license VARCHAR2(20)); / |
Here’s a query to show the hierarchy of object types and attributes by object-level in the hierarchy:
COL type_name FORMAT A20 HEADING TYPE_NAME COL attr_no FORMAT 999 HEADING ATTR_NO COL attr_name FORMAT A20 HEADING ATTR_NAME COL TYPE FORMAT A12 HEADING TYPE SELECT DISTINCT LPAD(' ',2*(LEVEL-1)) || ut.type_name AS type_name , uta.attr_no , uta.attr_name , CASE WHEN uta.attr_type_name = 'NUMBER' THEN uta.attr_type_name WHEN uta.attr_type_name = 'VARCHAR2' THEN uta.attr_type_name || '(' || uta.LENGTH || ')' END AS TYPE FROM user_types ut , user_type_attrs uta WHERE ut.typecode = 'OBJECT' AND ut.type_name = uta.type_name AND uta.inherited = 'NO' START WITH ut.type_name = 'BASE_T' CONNECT BY PRIOR ut.type_name = ut.supertype_name ORDER BY uta.attr_no; |
It should return the following:
TYPE_NAME ATTR_NO ATTR_NAME TYPE
-------------------- ------- -------------------- ------------
BASE_T 1 BASE_ID NUMBER
PERSON_T 2 FIRST_NAME VARCHAR2(20)
PERSON_T 3 MIDDLE_NAME VARCHAR2(20)
PERSON_T 4 LAST_NAME VARCHAR2(20)
DRIVER_T 5 LICENSE VARCHAR2(20) |
As always, I hope this helps those looking to discover an Oracle object type hierarchy without examining each object type in turn.
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