Archive for the ‘MySQL’ Category
Alice and Assignments
As I continue down the warren hole of Persistent Stored Modules (SQL/PSM) in MySQL, I keep wondering about that mad hare, Johnny Depp. Alice isn’t a programming language to teach me anything in this dream. Moreover, TIm Burton’s tale this seems oddly familiar, like a child’s story gone mad.
A quick update on comparative SQL expression assignments between PL/SQL and MySQL. When you want to filter a value through SQL functions before assigning it to another variable in MySQL, it’s not like PL/SQL. Just like the new Alice in Wonderland movie isn’t like the book.
The programmatic differences lies in their origins. PL/SQL evolved from Pascal through Ada to become a recursive language where you can call SQL from PL/SQL and PL/SQL from SQL. MySQL implemented PSMs from the ANSI SQL:2003 specification, which didn’t see it the same way, apparently (a disclaimer since I’ve not read the details of the specification).
Personally, I think PL/SQL is easier to write but I’ve been using it for almost 20 years. Naturally, there may be a consistency thread on this that I’m missing and an opportunity that I may exploit. After all, it is dark in this warren hole.
Oracle PL/SQL Assignments from SQL Expressions
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | -- Enable output printing. SET SERVEROUTPUT ON SIZE 1000000 -- Define an anonymous block. DECLARE -- Declare a source variable. lv_right_operand VARCHAR2(10) := 'March'; -- Define a target variable for the assignment. lv_left_operand VARCHAR2(10); BEGIN -- Return the expression from a nested call parameter of the source variable. lv_left_operand := UPPER(SUBSTR(lv_right_operand,1,3)); -- Print it to console. dbms_output.put_line('Output ['||lv_left_operand||']'); END; / |
Oracle also supports this syntax, which isn’t frequently used because it’s much more verbose syntactically. It is also equivalent to the PSM syntax adopted by MySQL.
-- Define an anonymous block. DECLARE -- Declare a source variable. lv_right_operand VARCHAR2(10) := 'March'; -- Define a target variable for the assignment. lv_left_operand VARCHAR2(10); BEGIN -- Return the expression from a nested call parameter of the source variable. SELECT UPPER(SUBSTR(lv_right_operand,1,3)) INTO lv_left_operand FROM dual; -- Print it to console. dbms_output.put_line('Output ['||lv_left_operand||']'); END; / |
That means we can do it like the White Queen wants it or the Red Queen wants it in Oracle. Flexibility in PL/SQL is clearly broader because of the assignment options. Not so in MySQL, as you’ll see.
MySQL PSM Assignment from SQL Expressions
First, MySQL’s PSM approach doesn’t support anonymous blocks. The example must create a stored function or procedure, and then call it. A procedure seems like the best fit for the example.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | -- Conditionally drop procedure. SELECT 'DROP PROCEDURE IF EXISTS assignit' AS "Statement"; DROP PROCEDURE IF EXISTS assignit; -- Create the proceudre SELECT 'CREATE PROCEDURE assignit' AS "Statement"; DELIMITER $$ -- Define the procedure. CREATE PROCEDURE assignit() BEGIN /* Declare a source variable. */ DECLARE lv_right_operand VARCHAR(10) DEFAULT 'March'; /* Define a target variable for the assignment. */ DECLARE lv_left_operand VARCHAR(3); /* Assign the modified value through the SELECT-INTO model. */ SELECT UCASE(SUBSTRING(lv_right_operand,1,3)) INTO lv_left_operand; /* Display assigned value. */ SELECT lv_left_operand; END; $$ DELIMITER ; -- Call the procedure. CALL assignit(); |
The only question here in the warren is: Who’s the White Queen; and who’s the Red Queen. Which semantic should I choose? My I hope is that I wake up before it’s … oops, off with his head. Actually, 3D or not, I’ll probably not see it, that’s the new Alice in Wonderland film.
Likewise, when my students wake up and read this they’ll know I was just answering a question on how to perform assignments in MySQL stored procedures. By the way, I’ve updated this assignment process in my Debugging MySQL Procedures post.
As an aside, I’ve got a new MySQL debugger that I’m testing later in the week. When I complete the test cases, I’ll post a review.
MySQL Standard Group By
Teaching SQL is interesting because folks try syntax that experience would tell you shouldn’t work. It was interesting when I discovered what should be broken from my perspective but was expected behavior in MySQL. It became clearer to me why it’s there as I did some experimenting with it enabled and disabled. While I’d still argue it’s broken, it’s the only way to get support for advanced aggregation concepts.
The reason that I find that it broken is a matter of perspective not standards. The fact that you can select a set of non-aggregated columns with an aggregated column, and exclude one, more than one, or all of the non-aggregated columns from the GROUP BY
clause seemed like a broken behavior. The MySQL behavior is explained in Chapter 11.12.3 of the MySQL 5.1 Reference. What wasn’t clear from the documentation when I read it was why the behavior is allowed. Gary’s comment on the original post made me look more deeply into the behavior.
If you check the referenced documentation, the process is called GROUP BY
hidden columns. That label didn’t immediately resonate with me. That’s probably of my background with IBM’s DB2 and Oracle. I’m so accustom to how they work, that sometimes I think they set the standards but they don’t. It turns out that the GROUP BY
clause in the ANSI SQL standards doesn’t allow for expressions.
The default configuration of MySQL allows for the expressions (functions) in the GROUP BY
clause. The documentation refers to hidden columns, which are in point of fact expressions in the GROUP BY
clause. MySQL does support expressions in the GROUP BY
by default, and requires, like Oracle, that when you use an expression in the SELECT
clause that you mirror it in the GROUP BY
clause. This means you group on the result of the expression, not a column in the table or view.
In the default configuration, you have three options. They are determinate results, indeterminate results, and hidden column results. The following cover these components.
Determinate Results
Determinate results are straight forward. They require that all non-aggregated columns in the SELECT
clause are mirrored in the GROUP BY
clause. This means that the non-aggregated column values are the key upon which results are aggregated.
An example of determinate results is:
1 2 3 4 5 6 | SELECT key_one , key_two , SUM(counter) FROM GROUPING GROUP BY key_one , key_two; |
Indeterminate Results
Indeterminate results isn’t straight forward but isn’t hard to grasp. An indeterminate result set is returned when one or more non-aggregated columns in a SELECT
clause aren’t listed in the GROUP BY
clause. The columns listed in the SELECT
clause but excluded from the GROUP BY
clause return meaningless values because they’re column values chosen indeterminately from all pre-aggregated rows.
The following query runs in a generically configured MySQL instance without an error. It returns a meaningless key_two
column value from the pre-aggregated row set. In the example, the aggregation column counts the unique key_one
column values. This behavior makes key_one
a determinate value, and key_two
an indeterminate value.
1 2 3 4 5 | SELECT key_one , key_two , SUM(counter) FROM GROUPING GROUP BY key_one; |
You can fix this mixed return set by adding the key_two
column to the GROUP BY
clause, which would return a determinate set. Alternatively, you can prevent the default behavior for the GROUP BY
clause by adding the ONLY_FULL_GROUP_BY
mode variable to your SQL_MODE
system variable.
After setting the SQL_MODE>
variable, a GROUP BY
must contain all non-aggregated columns. When you make this change to the SQL_MODE
system variable, you also disable any queries that use an expression in their GROUP BY
clause. Those queries with expressions in the group by will now raise an ERROR 1055
exception, like this:
ERROR 1055 (42000): 'sampledb.grouping.key_two' isn't in GROUP BY |
If you want to prevent indeterminate results and don’t use expressions in the GROUP BY
clause, you can add the ONLY_FULL_GROUP_BY
mode to your SQL_MODE
system variable. You can do that during a session with the following syntax:
SET SQL_MODE=(SELECT CONCAT(@@sql_mode,',ONLY_FULL_GROUP_BY')); |
Or, you can start the mysqld
with the following option:
mysqld --sql_mode="sql_mode1,sql_mode2, ... ,sql_mode(n+1)" |
A better alternative, is to add it to the my.cnf
configuration file on Linux, or my.ini
configuration file on Windows. You can add it to this line, which is done at the end of the line.
# Set the SQL mode to strict sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION,ONLY_FULL_GROUP_BY" |
If you opt for changing it at the server configuration file, you must stop and restart the mysqld
process. You can do that on Windows from the command line, like this on Windows provide the service name is mysql
. If you’ve set the Windows service to mysql51
, then you need to substitute mysql51
for mysql
.
Hidden Column Results
A hidden column result, is a result generated by an expression in the GROUP BY
clause. The following is a query that lets you sum transactions by the month name. This is supported by the default behavior of hidden columns. The following is an approach that you might find in Oracle because they don’t support aggregation by only part of the non-aggregated columns in a query.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | SELECT il.month AS "MON-YEAR" , il.base AS "BASE" , il.plus10 AS "10_PLUS" , il.plus20 AS "20_PLUS" FROM (SELECT CONCAT(UPPER(SUBSTRING(MONTHNAME(t.transaction_date),1,3)),'-',EXTRACT(YEAR FROM t.transaction_date)) AS MONTH , MONTH(t.transaction_date) AS sortkey , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.0,2)),10,' ') AS base , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.1,2)),10,' ') AS plus10 , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.2,2)),10,' ') AS plus20 FROM TRANSACTION t WHERE EXTRACT(YEAR FROM t.transaction_date) = 2009 GROUP BY CONCAT(UPPER(SUBSTRING(MONTHNAME(t.transaction_date),1,3)),'-',EXTRACT(YEAR FROM t.transaction_date)) , MONTH(t.transaction_date)) il ORDER BY il.sortkey; |
The interesting thing about MySQL’s approach is that the Oracle equivalent of an inline view is unnecessary. You can simply switch the functions like the exmaple below.
1 2 3 4 5 6 7 8 9 10 | SELECT DATE_FORMAT(t.transaction_date,'%m-%Y') AS MONTH , MONTH(t.transaction_date) AS sortkey , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.0,2)),10,' ') AS base , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.1,2)),10,' ') AS plus10 , LPAD(CONCAT('$',FORMAT(SUM(t.transaction_amount) * 1.2,2)),10,' ') AS plus20 FROM TRANSACTION t WHERE EXTRACT(YEAR FROM t.transaction_date) = 2009 GROUP BY DATE_FORMAT(t.transaction_date,'%m-%Y') , MONTH(t.transaction_date) ORDER BY sortkey; |
When you add the ONLY_FULL_GROUP_BY
mode to your SQL_MODE
system variable, you disallow this type of behavior and the query would no longer work. It would return the following error:
ERROR 1055 (42000): 'sampledb.t.transaction_date' isn't in GROUP BY |
Complete Code Sample ↓
Expand this section to see the sample working code for indeterminate results.
This builds the GROUPING
table, inserts nine rows, and tests it before and after setting the variable in the session. The change reverts after you exit and re-enter the database.
-- Conditionally drop sample table. SELECT 'DROP TABLE IF EXISTS grouping' AS "Statement"; DROP TABLE IF EXISTS GROUPING; -- Create sample table. SELECT 'CREATE TABLE grouping' AS "Statement"; CREATE TABLE GROUPING ( key_one CHAR(5) , key_two CHAR(5) , counter INT UNSIGNED ); -- Insert nine rows into the sample table. SELECT 'INSERT INTO grouping' AS "Statement" INSERT INTO GROUPING VALUES ('One','Uno-1',1),('Two','Due-1',2),('Three','Tre-1',3) ('One','Uno-2',1),('Two','Due-2',2),('Three','Tre-2',3) ('One','Uno-3',1),('Two','Due-3',2),('Three','Tre-3',3); -- Query with hidden columns, no enforcement on non-aggregate columns. SELECT 'SELECT non-aggregates FROM grouping with hidden columns' AS "Statement"; SELECT key_one, key_two, SUM(counter) FROM GROUPING GROUP BY key_one; -- Check current system mode variable assignment. SELECT 'SELECT @@sql_mode' AS "Statement"; SELECT @@sql_mode; -- Append the mode to the existing system mode variable. SELECT 'SET SQL_MODE=(SELECT ...)' AS "Statement"; SET SQL_MODE=(SELECT CONCAT(@@sql_mode,',ONLY_FULL_GROUP_BY')); -- Check modified system mode variable assignment. SELECT 'SELECT @@sql_mode' AS "Statement"; SELECT @@sql_mode; |
Debugging MySQL Procedures
In my second database class we focus on PL/SQL but we’ve begun highlighting the alternatives in MySQL and SQL Server. A student asked how they could debug runtime variable values in a MySQL Stored Procedure (or subroutines according to some documentation). You can see this post for how to create an equivalent solutions for MySQL functions.
In Oracle, we debug with the DBMS_OUTPUT
package. Packages, like DBMS_OUTPUT
hold related functions and procedures, and are a corollary to System.out.println()
in Java.
Before you can see the output at the command-line in Oracle (that is if you’re not using SQL*Developer or Toad), you must set a SQL*Plus environment variable. These variables don’t exist in MySQL or SQL Server command-line tools because they never served the function of a report writer like SQL*Plus.
You enable output display in Oracle by setting this in SQL*Plus:
SQL> SET SERVEROUTPUT ON SIZE 1000000 |
You can test your anonymous or named block. Since MySQL doesn’t support anonymous named block, the examples using a trivial procedure that prints Hello World! (orginal, right ;-)).
1 2 3 4 5 6 7 8 9 10 11 12 | -- Create a procedure in Oracle. CREATE OR REPLACE PROCEDURE hello_world IS BEGIN -- Print a word without a line return. DBMS_OUTPUT.put('Hello '); -- Print the rest of the phrase and a line return. DBMS_OUTPUT.put_line('World!'); END; / -- Call the procedure. EXECUTE hello_world; |
It’s seems useless to print the output because it should be evident. MySQL procedures are a bit different because there’s no OR REPLACE
syntax. The equivalent to calling the DBMS_OUTPUT
package procedures in MySQL is to simply select a string. Now you can do this with or without the FROM dual
clause in MySQL, don’t we wish we could do the same thing in Oracle. 🙂
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | -- Conditionally drop the procedure. SELECT 'DROP PROCEDURE hello_world' AS "Statement"; DROP PROCEDURE IF EXISTS hello_world; -- Reset the delimiter to write a procedure. DELIMITER $$ -- Create a procedure in Oracle. CREATE PROCEDURE hello_world() BEGIN -- Print the phrase and a line return. SELECT 'Hello World!'; END; $$ -- Reset the delimiter back to a semicolon to work again. DELIMITER ; -- Call the procedure. SELECT 'CALL hello_world' AS "Statement"; CALL hello_world(); |
Originally, I tried to keep this short but somebody wanted an example in a loop. Ouch, loops are so verbose in MySQL. Since I was modifying this post, it seemed like a good idea to put down some guidelines for successful development too.
Guidelines for Development of Procedures
Declaration Guidelines
The sequencing of components in MySQL procedures is important. Unlike, PL/SQL, there’s no declaration block, declarations must be at the top of the execution block. They also must appear in the following order:
- Variable declarations must go first, you can assign initial values with the
DEFAULT
keyword. While not required, you should:
- Consider using something like
lv_
to identify them as local variables for clarity and support of your code. - Consider grouping local variables that relate to handlers at the bottom of the list of variables.
- After local variables and before handlers, you put your cursor definitions. You should note that MySQL doesn’t support explicit dynamic cursors, which means you can’t define one with a formal signature. However, you do have prepared statements and they mimic dynamic cursor behaviors.
- Last in your declaration block, you declare your handler events.
Execution Guidelines
- Variable assignments are made one of two ways:
- You should start each execution block with a
START TRANSACTION
and then aSAVEPOINT
, which ensures the procedure acts like a cohesive programming unit. - You assign a
left_operand = right_operand;
as a statement. - You use the
SELECT column INTO variable;
syntax to filter a value through SQL functions and assign the resulting expression to a local variable. - You assign a single row cursor output to variables using a
SELECT column INTO variable FROM ...
.
- You must assign values from cursors called in a loop into local variables when you want to use the results in nested SQL statements or loops.
- You must reset looping variables, like the
fetched
control variable at the end of the loop to reuse the handler variable in subsequent loops. - You must assign values to local variables if you want to use them in the exception handler.
- If you’ve started a transaction, don’t forget to
COMMIT
your work.
Exception Guidelines
- Leave out the exception handler until you’ve tested all outcomes, and make sure you document them and add them as potential handlers.
- When you deploy exception blocks, they’re the last element at the bottom of the exception block.
- You should consider explicit exception handlers for each error unless the action taken is the same.
- You should consider grouping all exception handlers when the action taken is the same.
- You should include a
ROLLBACK
whenever you’ve performed two or more SQL statements that may modify data.
Below is an example for putting debug code inside a loop.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 | -- Conditionally drop a sample table. SELECT 'DROP TABLE IF EXISTS sample' AS "Statement"; DROP TABLE IF EXISTS sample; -- Create a table. CREATE TABLE sample ( sample_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT , sample_msg VARCHAR(20)); -- Insert into sample. INSERT INTO sample (sample_msg) VALUES ('Message #1') ,('Message #2') ,('Message #3'); -- Conditionally drop the procedure. SELECT 'DROP PROCEDURE debug_loop' AS "Statement"; DROP PROCEDURE IF EXISTS debug_loop; -- Reset the delimiter to write a procedure. DELIMITER $$ -- Create a procedure in Oracle. CREATE PROCEDURE debug_loop() BEGIN /* Declare a counter variable. */ DECLARE lv_counter INT DEFAULT 1; /* Declare local control loop variables. */ DECLARE lv_sample_id INT; DECLARE lv_sample_msg VARCHAR(20); /* Declare a local variable for a subsequent handler. */ DECLARE duplicate_key INT DEFAULT 0; DECLARE fetched INT DEFAULT 0; /* Declare a SQL cursor fabricated from local variables. */ DECLARE sample_cursor CURSOR FOR SELECT * FROM sample; /* Declare a duplicate key handler */ DECLARE CONTINUE HANDLER FOR 1062 SET duplicate_key = 1; /* Declare a not found record handler to close a cursor loop. */ DECLARE CONTINUE HANDLER FOR NOT FOUND SET fetched = 1; /* Start transaction context. */ START TRANSACTION; /* Set savepoint. */ SAVEPOINT all_or_none; /* Open a sample cursor. */ OPEN sample_cursor; cursor_sample: LOOP /* Fetch a row at a time. */ FETCH sample_cursor INTO lv_sample_id , lv_sample_msg; /* Place the catch handler for no more rows found immediately after the fetch operation. */ IF fetched = 1 THEN LEAVE cursor_sample; END IF; -- Print the cursor values. SELECT CONCAT('Row #',lv_counter,' [',lv_sample_id,'][',lv_sample_msg,']') AS "Rows"; -- Increment counter variable. SET lv_counter = lv_counter + 1; END LOOP cursor_sample; CLOSE sample_cursor; /* This acts as an exception handling block. */ IF duplicate_key = 1 THEN /* This undoes all DML statements to this point in the procedure. */ ROLLBACK TO SAVEPOINT all_or_none; END IF; END; $$ -- Reset the delimiter back to a semicolon to work again. DELIMITER ; -- Call the procedure. SELECT 'CALL debug_loop' AS "Statement"; CALL debug_loop(); |
This post certainly answers the student question. Hopefully, it also helps other who must migrate Oracle skills to MySQL. Since IBM DB2 has introduced a PL/SQL equivalent, wouldn’t it be nice if Oracle did that for MySQL. That is, migrate PL/SQL to MySQL. Don’t tell me if you think that’s a pipe dream, I’d like to hope for that change.
MySQL Boolean Data Type
Somebody posted a quick question about the outcome of defining a table with a bool
data type in PHPMyAdmin. They were subsequently surprised when they checked the MySQL database and found that it was actually a tinyint(1)
. The natural question they had was: “What do you enter – true/false or 1/0?”
I promised to post an answer tonight, and morning counts too. You can enter a true
or false
because they’re synonyms for a 1
or 0
respectively. TINYINT
is the supported data type, and BIT
, BOOL
, and BOOLEAN
are synonyms for the base data type.
Here’s an example in MySQL:
mysql> CREATE TABLE data_type (TYPE bool); mysql> DESCRIBE data_type; +-------+------------+------+-----+---------+-------+ | FIELD | TYPE | NULL | KEY | DEFAULT | Extra | +-------+------------+------+-----+---------+-------+ | TYPE | tinyint(1) | YES | | NULL | | +-------+------------+------+-----+---------+-------+ mysql> INSERT INTO data_type VALUES (TRUE),(FALSE); mysql> SELECT * FROM data_type; +------+ | TYPE | +------+ | 1 | | 0 | +------+ |
The comment below raises the question of what happens with values in the valid range of TINYINT
that aren’t 0
or 1
, like 5. The simple answer is they’re not valid when compared against the true
and false
constants, as you can see by creating the following example.
-- Create a test table. CREATE TABLE verify ( verify_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT , range_1 tinyint UNSIGNED , range_2 tinyint ); -- Insert test values. INSERT INTO verify VALUES (NULL, 0, 0) ,(NULL, 1, 1) ,(NULL, 1,-1) ,(NULL, 2, 2); -- Query results. SELECT range_1 AS "Value" , CASE WHEN range_1 = TRUE THEN 'True' WHEN range_1 = FALSE THEN 'False' ELSE 'Invalid' END AS "Unsigned" , range_2 AS "Value" , CASE WHEN range_2 = TRUE THEN 'True' WHEN range_2 = FALSE THEN 'False' ELSE 'Invalid' END AS "Signed" FROM verify; |
The results of the test demonstrate that only a 0
or 1
value validates against the false
or true
constants, as shown:
+-------+----------+-------+---------+ | Value | Unsigned | Value | Signed | +-------+----------+-------+---------+ | 0 | False | 0 | False | | 1 | True | 1 | True | | 1 | True | -1 | Invalid | | 2 | Invalid | 2 | Invalid | +-------+----------+-------+---------+ |
Based on the comment, the 256 value range of TINYINT
is found here.
SQL Aggregation Tutorial
I’ve been working on a Basic Aggregation tutorial for my students. I think this might be close to what may benefit them. However, I thought it would be great to put it out there and solicit ideas. If you have some on improving this post, please let me know.
My first take at the post …
This is a lesson on basic aggregation in SQL. Aggregation in SQL means counting, adding, and grouping by results of counts or sums. Aggregation is a critical part of using the SQL language. At a basic level, aggregation includes the COUNT
, SUM
, AVERAGE
, MAX
, and MIN
aggregation functions; and the ORDER BY
, GROUP BY
, and HAVING
clauses.
You’ll find the setup scripts for these examples at the bottom of this blog page. The best way to use this page is to copy the setup code, run it in your database, and then test the examples as you work though them.
Data set ↓
This section discusses the data set. You unfold it by clicking on the bold text above.
The illustration of aggregation is a challenge because a trivial set of numbers doesn’t present much challenge or sometimes relevance but it shows the basic concept. A more applied sample set makes mental calculation and comprehension of functions more difficult. As a result of that trade off, this uses two copies of the ordinal numbers (ordinal numbers are the single digit integers from zero to nine).
After you run the setup script, you can display the data set with the following query.
-- Oracle only SQL*Plus formatting command to smooth output display. SET PAGESIZE 99 -- Query ordered by the sets and then the integers in ascending order. SELECT * FROM ordinal ORDER BY 2, 4; |
You could have sorted them more easily by using the ORDINAL_ID
surrogate key column but it wouldn’t demonstrate the ORDER BY
clause. In some database, like Oracle and MySQL, the ORDER BY
lets you use column names, column aliases, or the position order of the column in the SELECT
clause. Unfortunately, this isn’t true across all databases.
The ORDER BY
clause does let you see that you can set nested ordering, like the numbers inside the data sets that are qualified by the LIST_SET
column. This is demonstrated in the output below.
ORDINAL_ID LIST_SET LIST_NAME LIST_VALUE ---------- -------------------- ---------- ---------- 1 Value Set A Zero 0 2 Value Set A One 1 3 Value Set A Two 2 4 Value Set A Three 3 5 Value Set A Four 4 6 Value Set A Five 5 7 Value Set A Six 6 8 Value Set A Seven 7 9 Value Set A Eight 8 10 Value Set A Nine 9 11 Value Set A 12 Value Set B Zero 0 13 Value Set B One 1 14 Value Set B Two 2 15 Value Set B Three 3 16 Value Set B Four 4 17 Value Set B Five 5 18 Value Set B Six 6 19 Value Set B Seven 7 20 Value Set B Eight 8 21 Value Set B Nine 9 22 Value Set B |
Single row aggregation function use ↓
This section discusses and demonstrates the five aggregation functions in the post. They return a single row of data. Like the other sections, you unfold this by clicking on the bold text above.
COUNT
function
The COUNT
function has two behaviors that may apply. One is when you count rows and the other is when you count values. Both return only a single row.
Counting by Reference: The following example demonstrates counting rows. In this scenario, the COUNT
function takes a single call parameter, which is an asterisk (*
). The asterisk actually counts the references to the rows in the table, and in Oracle is equivalent to COUNT(ROWID)
. The pseudo column ROWID
points to the physical block address where the row is stored in the database. As such, the asterisk acts very much like a pointer in the C or C++ language (that is if you substitute a block address for a memory address).
SELECT COUNT(*) FROM ordinal; |
It returns
COUNT(*) ---------- 22 |
When you count rows, you count them whether they contain values or not. It is possible that you could insert a meaningless row with null values for all columns in a table that’s unconstrained. Moreover, the COUNT
function counts all rows no matter whether contain any values.
Counting by Value: The following examples demonstrate approaches to counting values. It’s important to note that when you count values, you ignore null values. You can also count distinct things or all things when you count by value.
The LIST_SET
column doesn’t contain any null values (as you can see by inspecting the data set early in this blog page). The following counts the number of values in the LIST_SET
column. You could also substitute COUNT(ALL list_set)
and it would return the same thing because ALL
is the default and always provided unless you override it with a DISTINCT
keyword.
SELECT COUNT(list_set) FROM ordinal; |
It returns:
COUNT(*) ---------- 22 |
Since the LIST_SET
column only contains one of two values, you can count that to make sure. You do it with the following syntax:
SELECT COUNT(DISTINCT list_set) FROM ordinal; |
It returns:
COUNT(*) ---------- 2 |
The LIST_NAME
and LIST_VALUE
both contain two null values. Let’s see what they return when we count all or distinct values. This example simplifies it a step more by performing both counts side-by-side. This is possible because both columns return a single row.
SELECT COUNT(ALL list_name) AS name_number , COUNT(DISTINCT list_value) AS value_number FROM ordinal; |
It returns the twenty total string values found in the first column and the ten unique numeric values found in the second column. The COUNT
function with the DISTINCT
filter acts like a COUNT
function with a GROUP BY
clause on the LIST_VALUE
column, as you’ll see later in the discussion.
NAME_NUMBER VALUE_NUMBER ----------- ------------ 20 10 |
SUM
, AVERAGE
, MAX
, and MIN
functions
Math Operations by Value: These math aggregations are done with the SUM
, AVERAGE
, MAX
, and MIN
aggregation functions. They’re restricted to columns that contain numeric values. Each of them support the default ALL
or DISTINCT
keywords and they behave much as the COUNT
function discussed earlier.
There is a significant difference between their functionality. You can count unique strings but you can’t sum a column of numbers based on that other column without a GROUP BY
clause. You see examples of the GROUP BY
in the next part of this post.
Since these behave more or less the same, their examples are grouped together in sections by summing and averaging together, and taking the maximum and minimum values together. For reference, the ordinal numbers sum to 45, two sets of them are naturally 90; and the average of evenly dispersed numbers is half, or 45, and their unique values 4.5.
SELECT SUM(ALL list_value) AS sum_all , SUM(DISTINCT list_value) AS sum_distinct , AVG(ALL list_value) AS avg_all , AVG(DISTINCT list_value) AS avg_distinct FROM ordinal; |
It returns:
SUM_ALL SUM_DISTINCT AVG_ALL AVG_DISTINCT ---------- ------------ ---------- ------------ 90 45 4.5 4.5 |
The next section makes the same comparison with the MAX
, and MIN
functions. The maximum or minimums are the same because they the uniqueness doesn’t change the minimum or maximum of the sets.
SELECT MIN(ALL list_value) AS min_all , MIN(DISTINCT list_value) AS min_distinct , MAX(ALL list_value) AS max_all , MAX(DISTINCT list_value) AS max_distinct FROM ordinal; |
It returns:
MIN_ALL MIN_DISTINCT MAX_ALL MAX_DISTINCT ---------- ------------ ---------- ------------ 0 0 9 9 |
Multiple row aggregation function use ↓
This section discusses and demonstrates the five aggregation functions where they return more than a single row of data, which involves grouping by non-aggregated columns. This section covers the GROUP BY
and HAVING
clauses. Like the other sections, you unfold this by clicking on the bold text above.
COUNT
function
Counting by Groups: The following example demonstrates how we can count rows or values by groups. The example counts rows by reference and value, but it does so by grouping the return values on the unique string values in the LIST_SET
column.
1 2 3 4 5 | SELECT list_set AS grouping_by_column , COUNT(*) , COUNT(list_value) FROM ordinal GROUP BY list_set; |
It returns the following values:
GROUPING_BY_COLUMN COUNT(*) COUNT(LIST_VALUE) -------------------- ---------- ----------------- VALUE SET A 11 10 VALUE SET B 11 10 |
The results show that there are eleven rows for each set, and that ten of the rows contain values while one holds a null value.
You can also group on columns that contain null values or sets of columns. The following example groups by two columns, where one contains a null value.
1 2 3 4 5 6 7 8 | SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , COUNT(*) , COUNT(list_value) FROM ordinal WHERE list_set = 'Value Set A' GROUP BY list_set , list_name; |
It returns the following values:
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO COUNT(*) COUNT(LIST_VALUE) -------------------- ----------------- ---------- ----------------- VALUE SET A Zero 1 1 VALUE SET A Five 1 1 VALUE SET A Three 1 1 VALUE SET A Four 1 1 VALUE SET A One 1 1 VALUE SET A Two 1 1 VALUE SET A Eight 1 1 VALUE SET A Nine 1 1 VALUE SET A Seven 1 1 VALUE SET A Six 1 1 VALUE SET A 1 0 |
The last row returned is the one where the grouping by column value and counted column value are null. You should note that the GROUP BY
applies to a string column and the return set includes a row grouped by its null value. The count of null column values is always zero.
Another thing that you may have not noticed is that the rows are no longer in ascending numeric order by LIST_NAME
or LIST_VALUE
columns. There’s a multiple edge sword when you examine why this occurs. Basically with regard to the LIST_NAME
column, Oracle and many other databases use advanced sorting algorithms which may leave non-ordered sets as outcomes. The LIST_VALUE
column is not in the GROUP BY
clause and therefore can’t be used as an ORDER BY
key.
When you attempt to use a column outside of the GROUP BY
clause to order the return set, like this:
1 2 3 4 5 6 7 8 9 | SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , COUNT(*) , COUNT(list_value) FROM ordinal WHERE list_set = 'Value Set A' GROUP BY list_set , list_name ORDER BY list_value; |
You would see the following error in an Oracle Database 10g instance:
ORDER BY list_value * ERROR at line 9: ORA-00979: NOT a GROUP BY expression |
You can filter this particular set because the conditions are limited, but this wouldn’t be a good idea in many other solution sets. However, it certainly highlights some of the potential for extraordinary sorting of result sets.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , COUNT(*) , COUNT(list_value) FROM ordinal WHERE list_set = 'Value Set A' GROUP BY list_set , list_name ORDER BY CASE WHEN list_name = 'Zero' THEN 0 WHEN list_name = 'One' THEN 1 WHEN list_name = 'Two' THEN 2 WHEN list_name = 'Three' THEN 3 WHEN list_name = 'Four' THEN 4 WHEN list_name = 'Five' THEN 5 WHEN list_name = 'Six' THEN 6 WHEN list_name = 'Seven' THEN 7 WHEN list_name = 'Eight' THEN 8 WHEN list_name = 'Nine' THEN 9 END; |
It returns the following values:
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO COUNT(*) COUNT(LIST_VALUE) -------------------- ----------------- ---------- ----------------- VALUE SET A Zero 1 1 VALUE SET A One 1 1 VALUE SET A Two 1 1 VALUE SET A Three 1 1 VALUE SET A Four 1 1 VALUE SET A Five 1 1 VALUE SET A Six 1 1 VALUE SET A Seven 1 1 VALUE SET A Eight 1 1 VALUE SET A Nine 1 1 VALUE SET A 1 0 |
A better way to sort this type of data would involve putting it into an inline view and then joining it against the base data set. This allows you to use a non-grouping column as the sort key in an ORDER BY
clause. Here’s an example of that approach but note it does take system overhead and should only be done when SQL must perform the sort operation.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | SELECT ilv.grouping_by_not_null , ilv.group_by_null_too , ilv.row_count , ilv.column_count FROM (SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , COUNT(*) AS ROW_COUNT , COUNT(list_value) AS column_count FROM ordinal WHERE list_set = 'Value Set A' GROUP BY list_set , list_name) ilv LEFT JOIN ordinal o ON ilv.grouping_by_not_null = o.list_set AND ilv.group_by_null_too = o.list_name ORDER BY o.list_value; |
That returns a perfectly ordered set because the LEFT JOIN
lets us capture the null value in the inline view without matching the two columns in the join condition. It returns the following set:
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO ROW_COUNT COLUMN_COUNT -------------------- ----------------- ---------- ------------ Value Set A Zero 1 1 Value Set A One 1 1 Value Set A Two 1 1 Value Set A Three 1 1 Value Set A Four 1 1 Value Set A Five 1 1 Value Set A Six 1 1 Value Set A Seven 1 1 Value Set A Eight 1 1 Value Set A Nine 1 1 Value Set A 1 0 |
SUM
, AVERAGE
, MAX
, and MIN
functions
Math Operations by Groups: These math aggregations are done with the SUM
, AVERAGE
, MAX
, and MIN
aggregation functions. Like the previous single row return examples, they’re restricted to columns that contain numeric values. While they also support the default ALL
or DISTINCT
keywords, it didn’t seem necessary to demonstrate more of those behaviors here.
The following sums, averages, and gets both the minimum and maximum value by unique LIST_SET
column values:
SELECT list_set AS grouping_by_not_null , SUM(list_value) AS ordinal_sum , AVG(list_value) AS ordinal_avg , MIN(list_value) AS ordinal_min , MAX(list_value) AS ordinal_max FROM ordinal GROUP BY list_set; |
It returns the following two rows:
GROUPING_BY_NOT_NULL ORDINAL_SUM ORDINAL_AVG ORDINAL_MIN ORDINAL_MAX -------------------- ----------- ----------- ----------- ----------- VALUE SET A 45 4.5 0 9 VALUE SET B 45 4.5 0 9 |
Expanding the GROUP BY
criteria to include the LIST_NAME
column, you see what happens when the SUM
and AVG
functions work with only null values. They always return null when adding only column values with a null value. This is handy as you’ll see in the selective aggregation discussion in a subsequent tutorial.
As done in the prior example, this filters out the second value set, to focus on what’s happening with the rows grouped on two columns.
1 2 3 4 5 6 7 8 9 10 | SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , SUM(list_value) AS ordinal_sum , AVG(list_value) AS ordinal_avg , MIN(list_value) AS ordinal_min , MAX(list_value) AS ordinal_max FROM ordinal WHERE NOT list_set = 'Value Set B' GROUP BY list_set , list_name; |
It returns the following set of records, which are clearly out of order based on the algorithms that optimize aggregation. It’s not hard to read because there aren’t too many rows returned.
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO ORDINAL_SUM ORDINAL_AVG ORDINAL_MIN ORDINAL_MAX -------------------- ----------------- ----------- ----------- ----------- ----------- VALUE SET A Zero 0 0 0 0 VALUE SET A Five 5 5 5 5 VALUE SET A Three 3 3 3 3 VALUE SET A Four 4 4 4 4 VALUE SET A One 1 1 1 1 VALUE SET A Two 2 2 2 2 VALUE SET A Eight 8 8 8 8 VALUE SET A Nine 9 9 9 9 VALUE SET A Seven 7 7 7 7 VALUE SET A Six 6 6 6 6 VALUE SET A |
If SQL must sort these into numeric order, it can do so like the prior example with the result from the COUNT
function. You must instrument a solution for it through an inline view. The following shows you how to do that.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | SELECT ilv.grouping_by_not_null , ilv.group_by_null_too , ilv.ordinal_sum , ilv.ordinal_avg , ilv.ordinal_min , ilv.ordinal_max FROM (SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , SUM(list_value) AS ordinal_sum , AVG(list_value) AS ordinal_avg , MIN(list_value) AS ordinal_min , MAX(list_value) AS ordinal_max FROM ordinal WHERE list_set = 'Value Set B' GROUP BY list_set , list_name) ilv LEFT JOIN ordinal o ON ilv.grouping_by_not_null = o.list_set AND ilv.group_by_null_too = o.list_name ORDER BY o.list_value; |
Now you should see an ordered set like this:
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO ORDINAL_SUM ORDINAL_AVG ORDINAL_MIN ORDINAL_MAX -------------------- ----------------- ----------- ----------- ----------- ----------- VALUE SET B Zero 0 0 0 0 VALUE SET B One 1 1 1 1 VALUE SET B Two 2 2 2 2 VALUE SET B Three 3 3 3 3 VALUE SET B Four 4 4 4 4 VALUE SET B Five 5 5 5 5 VALUE SET B Six 6 6 6 6 VALUE SET B Seven 7 7 7 7 VALUE SET B Eight 8 8 8 8 VALUE SET B Nine 9 9 9 9 VALUE SET B |
Math Operations with the HAVING
clause: The HAVING
clause is very useful when you want to find rows that duplicate a column or set of column values. It’s also useful for sorting data sets.
The HAVING
clause effects all aggregate rows and acts somewhat like a filter in the WHERE
clause of a non-aggregated query. The difference is that the WHERE
clause filters the base result set, while the HAVING
clause filters the aggregated result set.
The following example captures the sum and average for only the odd LIST_VALUE
column values. It does that by using modular (or modulo) mathematics through the MOD
function in Oracle’s SQL implementation. Modulo math works by calculating the remainder of integer division. When you divide any number by two and the result is one, then you have an odd number.
Here’s the sample program that illustrates the HAVING
clause in a CASE
statement. The CASE
statement only adds the number when it is an odd number. Any even number is zeroed out. The comparison operator ensures that only rows are returned when the SUM
is greater than zero, which only happens for odd numbers.
1 2 3 4 5 6 7 8 9 10 11 | SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , SUM(list_value) AS ordinal_sum , AVG(list_value) AS ordinal_avg FROM ordinal WHERE list_set = 'Value Set A' HAVING SUM(CASE WHEN MOD(list_value,2) = 1 THEN list_value ELSE 0 END) > 0 GROUP BY list_set , list_name; |
The data set, as you’ve seen with others isn’t sorted in ascending order because of how the algorithms work. Since there are only five rows returned, it is easy to see that the HAVING
clause let us narrow the return set to odd numbers. What we might overlook is that the HAVING
is always a filtering statement. This means we need to have a comparative operator tied to the HAVING
return value.
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO ORDINAL_SUM ORDINAL_AVG -------------------- ----------------- ----------- ----------- Value Set A Five 5 5 Value Set A Three 3 3 Value Set A One 1 1 Value Set A Nine 9 9 Value Set A Seven 7 7 |
As you’ve seen in early examples, you can also put this inside an inline view and sort the aggregated results. Here’s the code to accomplish that.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | SELECT ilv.grouping_by_not_null , ilv.group_by_null_too , ilv.ordinal_sum , ilv.ordinal_avg FROM (SELECT list_set AS grouping_by_not_null , list_name AS group_by_null_too , SUM(list_value) AS ordinal_sum , AVG(list_value) AS ordinal_avg FROM ordinal WHERE list_set = 'Value Set A' HAVING SUM(CASE WHEN MOD(list_value,2) = 1 THEN list_value ELSE 0 END) > 0 GROUP BY list_set , list_name) ilv LEFT JOIN ordinal o ON ilv.grouping_by_not_null = o.list_set AND ilv.group_by_null_too = o.list_name ORDER BY o.list_value; |
It naturally returns the ordered set of aggregated results from only odd numbers in the base set of values.
GROUPING_BY_NOT_NULL GROUP_BY_NULL_TOO ORDINAL_SUM ORDINAL_AVG -------------------- ----------------- ----------- ----------- Value Set A One 1 1 Value Set A Three 3 3 Value Set A Five 5 5 Value Set A Seven 7 7 Value Set A Nine 9 9 |
Setup Script ↓
The drop down items, unfold with source code to seed the examples.
Oracle SQL Join Supporting Script ↓
This has the setup script for the example tables.
-- Conditionally drop the table and sequence. BEGIN FOR i IN (SELECT TABLE_NAME FROM user_tables WHERE TABLE_NAME = 'ORDINAL') LOOP EXECUTE IMMEDIATE 'DROP TABLE '||i.table_name; END LOOP; FOR i IN (SELECT sequence_name FROM user_sequences WHERE sequence_name = 'ORDINAL_S1') LOOP EXECUTE IMMEDIATE 'DROP SEQUENCE '||i.sequence_name; END LOOP; END; / -- Create the aggregation sample table and sequence. CREATE TABLE ordinal ( ordinal_id NUMBER , list_set VARCHAR2(20) , list_name VARCHAR2(5) , list_value NUMBER); CREATE SEQUENCE ordinal_s1; -- Seeding values in the aggregation sample table. DECLARE -- Define local types. TYPE listg IS TABLE OF VARCHAR2(20); TYPE listn IS TABLE OF NUMBER; TYPE lists IS TABLE OF VARCHAR2(5); -- Declare local variables. groups LISTG := listg('Value Set A','Value Set B'); labels LISTS := lists('Zero','One','Two','Three','Four','Five','Six','Seven','Eight','Nine'); valuen LISTN := listn(0,1,2,3,4,5,6,7,8,9); BEGIN -- Outer loop sets the group level. FOR i IN 1..groups.COUNT LOOP -- Inner loop sets the row level. FOR j IN 1..labels.COUNT LOOP INSERT INTO ordinal VALUES (ordinal_s1.NEXTVAL,groups(i),labels(j),valuen(j)); END LOOP; -- Insert the null values for each group. INSERT INTO ordinal VALUES (ordinal_s1.NEXTVAL,groups(i),NULL,NULL); END LOOP; COMMIT; END; / -- Commit the inserts. COMMIT; |
MySQL SQL Join Supporting Script ↓
This has the setup script for the example tables.
-- Conditionally drop the table. SELECT 'DROP TABLE ordinal' AS Statement; DROP TABLE IF EXISTS ordinal; -- Create the table. SELECT 'CREATE TABLE ordinal' AS Statement; CREATE TABLE ordinal ( ordinal_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT , list_set VARCHAR(20) , list_name VARCHAR(5) , list_value INT UNSIGNED); -- Seed the row values. SELECT 'INSERT INTO ordinal' AS Statement; INSERT INTO ordinal VALUES (NULL,'Value Set A','Zero','0') ,(NULL,'Value Set A','One','1') ,(NULL,'Value Set A','Two','2') ,(NULL,'Value Set A','Three','3') ,(NULL,'Value Set A','Four','4') ,(NULL,'Value Set A','Five','5') ,(NULL,'Value Set A','Six','6') ,(NULL,'Value Set A','Seven','7') ,(NULL,'Value Set A','Eight','8') ,(NULL,'Value Set A','Nine','9') ,(NULL,'Value Set A',NULL,NULL) ,(NULL,'Value Set B','Zero','0') ,(NULL,'Value Set B','One','1') ,(NULL,'Value Set B','Two','2') ,(NULL,'Value Set B','Three','3') ,(NULL,'Value Set B','Four','4') ,(NULL,'Value Set B','Five','5') ,(NULL,'Value Set B','Six','6') ,(NULL,'Value Set B','Seven','7') ,(NULL,'Value Set B','Eight','8') ,(NULL,'Value Set B','Nine','9') ,(NULL,'Value Set B',NULL,NULL); -- Commit the inserts. COMMIT; |
Multi-row Merge in MySQL
After I wrote the post for students on the multiple row MERGE
statement for an upload through an external table in Oracle, I thought to check how it might be done with MySQL. More or less because I try to keep track of how things are done in several databases.
MySQL’s equivalent to a MERGE
statement is an INSERT
statement with an ON DUPLICATE KEY
clause, which I blogged about a while back. You may also use the REPLACE INTO
when you want to merge more than one row. At the time that I wrote this, I thought there wasn’t support for an INSERT ON DUPLICATE KEY
clause statement with a subquery but I found that I was wrong. Fortunately, somebody posted a comment to remind me about this and now both solutions are here for anybody that would like them.
The workaround with a VALUES
clause was to write a stored procedure with two cursor loops, explicitly pass the values from the cursor to local variables, and then put the local variables in the VALUES
clause. I’ll post the other with a subquery soon. On parity, clearly Oracle’s MERGE
statement (shown here) is far superior than MySQL’s approach.
Demonstration
Here are the steps to accomplish an import/upload with the INSERT
statement and ON DUPLICATE KEY
clause. In this example, you upload data from a flat file, or Comma Separated Value (CSV) file to a denormalized table (actually in unnormalized form). This type of file upload transfers information that doesn’t have surrogate key values. You have to create those in the scope of the transformation to the normalized tables.
Step #1 : Position your CSV file in the physical directory
After creating the virtual directory, copy the following contents into a file named kingdom_mysql_import.csv
in the C:\Data\Download
directory or folder. If you have Windows UAC enabled in Windows Vista or 7, you should disable it before performing this step.
Place the following in the kingdom_mysql_import.csv
file. The trailing commas are meaningful in MySQL and avoid problems when reading CSV files.
Narnia, 77600,'Peter the Magnificent',12720320,12920609, Narnia, 77600,'Edmund the Just',12720320,12920609, Narnia, 77600,'Susan the Gentle',12720320,12920609, Narnia, 77600,'Lucy the Valiant',12720320,12920609, Narnia, 42100,'Peter the Magnificent',15310412,15310531, Narnia, 42100,'Edmund the Just',15310412,15310531, Narnia, 42100,'Susan the Gentle',15310412,15310531, Narnia, 42100,'Lucy the Valiant',15310412,15310531, Camelot, 15200,'King Arthur',06310310,06861212, Camelot, 15200,'Sir Lionel',06310310,06861212, Camelot, 15200,'Sir Bors',06310310,06351212, Camelot, 15200,'Sir Bors',06400310,06861212, Camelot, 15200,'Sir Galahad',06310310,06861212, Camelot, 15200,'Sir Gawain',06310310,06861212, Camelot, 15200,'Sir Tristram',06310310,06861212, Camelot, 15200,'Sir Percival',06310310,06861212, Camelot, 15200,'Sir Lancelot',06700930,06821212, |
Step #2 : Connect as the student
user
Disconnect and connect as the student user, or reconnect as the student user. The reconnect syntax that protects your password is:
mysql -ustudent -p |
Connect to the sampledb
database, like so:
mysql> USE sampledb; |
Step #3 : Run the script that creates tables and sequences
Copy the following into a create_mysql_kingdom_upload.sql
file within a directory of your choice. Then, run it as the student
account.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 | -- This enables dropping tables with foreign key dependencies. -- It is specific to the InnoDB Engine. SET FOREIGN_KEY_CHECKS = 0; -- Conditionally drop objects. SELECT 'KINGDOM' AS "Drop Table"; DROP TABLE IF EXISTS KINGDOM; SELECT 'KNIGHT' AS "Drop Table"; DROP TABLE IF EXISTS KNIGHT; SELECT 'KINGDOM_KNIGHT_IMPORT' AS "Drop Table"; DROP TABLE IF EXISTS KINGDOM_KNIGHT_IMPORT; -- Create normalized kingdom table. SELECT 'KINGDOM' AS "Create Table"; CREATE TABLE kingdom ( kingdom_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT , kingdom_name VARCHAR(20) , population INT UNSIGNED) ENGINE=INNODB; -- Create normalized knight table. SELECT 'KNIGHT' AS "Create Table"; CREATE TABLE knight ( knight_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT , knight_name VARCHAR(24) , kingdom_allegiance_id INT UNSIGNED , allegiance_start_date DATE , allegiance_end_date DATE , CONSTRAINT fk_kingdom FOREIGN KEY (kingdom_allegiance_id) REFERENCES kingdom (kingdom_id)) ENGINE=INNODB; -- Create external import table in memory only - disappears after rebooting the mysqld service. SELECT 'KINGDOM_KNIGHT_IMPORT' AS "Create Table"; CREATE TABLE kingdom_knight_import ( kingdom_name VARCHAR(20) , population INT UNSIGNED , knight_name VARCHAR(24) , allegiance_start_date DATE , allegiance_end_date DATE) ENGINE=MEMORY; |
Step #4 : Load the data into your target upload table
There a number of things that could go wrong but when you choose LOCAL
there generally aren’t any problems. Run the following query from the student
account while using the sampledb
database, and check whether or not you can access the kingdom_import.csv
file.
1 2 3 4 5 6 | LOAD DATA LOCAL INFILE 'c:/Data/kingdom_mysql_import.csv' INTO TABLE kingdom_knight_import FIELDS TERMINATED BY ',' ENCLOSED BY '"' ESCAPED BY '\\' LINES TERMINATED BY '\r\n'; |
Step #5 : Create the upload procedure
Copy the following into a create_mysql_upload_procedure.sql
file within a directory of your choice. You should note that unlike Oracle’s MERGE
statement, this is done with the ON DUPLICATE KEY
clause and requires actual values not a source query. This presents few options other than a stored routine, known as a stored procedure. As you can see from the code, there’s a great deal of complexity to the syntax and a much more verbose implementation than Oracle’s equivalent PL/SQL.
Then, run it as the student
account. As you look at the structure to achieve this simple thing, the long standing complaint about PL/SQL being a verbose language comes to mind. Clearly, stored procedures are new to MySQL but they’re quite a bit more verbose than PL/SQL.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 | -- Conditionally drop the procedure. SELECT 'UPLOAD_KINGDOM' AS "Drop Procedure"; DROP PROCEDURE IF EXISTS upload_kingdom; -- Reset the execution delimiter to create a stored program. DELIMITER $$ -- The parentheses after the procedure name must be there or the MODIFIES SQL DATA raises an compile time exception. CREATE PROCEDURE upload_kingdom() MODIFIES SQL DATA BEGIN /* Declare local variables. */ DECLARE lv_kingdom_id INT UNSIGNED; DECLARE lv_kingdom_name VARCHAR(20); DECLARE lv_population INT UNSIGNED; DECLARE lv_knight_id INT UNSIGNED; DECLARE lv_knight_name VARCHAR(24); DECLARE lv_kingdom_allegiance_id INT UNSIGNED; DECLARE lv_allegiance_start_date DATE; DECLARE lv_allegiance_end_date DATE; /* Declare a handler variables. */ DECLARE duplicate_key INT DEFAULT 0; DECLARE foreign_key INT DEFAULT 0; DECLARE fetched INT DEFAULT 0; /* Cursors must come after variables and before event handlers. */ /* Declare a SQL cursor with a left join on the natural key. */ DECLARE kingdom_cursor CURSOR FOR SELECT DISTINCT k.kingdom_id , kki.kingdom_name , kki.population FROM kingdom_knight_import kki LEFT JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population; /* Declare a SQL cursor with a join on the natural key. */ DECLARE knight_cursor CURSOR FOR SELECT kn.knight_id , kki.knight_name , k.kingdom_id , kki.allegiance_start_date AS start_date , kki.allegiance_end_date AS end_date FROM kingdom_knight_import kki INNER JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population LEFT JOIN knight kn ON k.kingdom_id = kn.kingdom_allegiance_id AND kki.knight_name = kn.knight_name AND kki.allegiance_start_date = kn.allegiance_start_date AND kki.allegiance_end_date = kn.allegiance_end_date; /* Event handlers must always be last in the declaration section. */ /* Declare a duplicate key handler */ DECLARE CONTINUE HANDLER FOR 1062 SET duplicate_key = 1; DECLARE CONTINUE HANDLER FOR 1216 SET foreign_key = 1; /* Declare a not found record handler to close a cursor loop. */ DECLARE CONTINUE HANDLER FOR NOT FOUND SET fetched = 1; /* ---------------------------------------------------------------------- */ /* Start transaction context. */ START TRANSACTION; /* Set savepoint. */ SAVEPOINT both_or_none; /* Open a local cursor. */ OPEN kingdom_cursor; cursor_kingdom: LOOP FETCH kingdom_cursor INTO lv_kingdom_id , lv_kingdom_name , lv_population; /* Place the catch handler for no more rows found immediately after the fetch operation. */ IF fetched = 1 THEN LEAVE cursor_kingdom; END IF; INSERT INTO kingdom VALUES ( lv_kingdom_id , lv_kingdom_name , lv_population ) ON DUPLICATE KEY UPDATE kingdom_name = lv_kingdom_name; END LOOP cursor_kingdom; CLOSE kingdom_cursor; /* Reset the continue handler to zero. */ SET fetched = 0; /* Open a local cursor. */ OPEN knight_cursor; cursor_knight: LOOP /* Fetch records until they're all read, and a NOT FOUND SET is returned. */ FETCH knight_cursor INTO lv_knight_id , lv_knight_name , lv_kingdom_allegiance_id , lv_allegiance_start_date , lv_allegiance_end_date; /* Place the catch handler for no more rows found immediately after the fetch operation. */ IF fetched = 1 THEN LEAVE cursor_knight; END IF; INSERT INTO knight VALUES ( lv_knight_id , lv_knight_name , lv_kingdom_allegiance_id , lv_allegiance_start_date , lv_allegiance_end_date ) ON DUPLICATE KEY UPDATE knight_name = lv_knight_name; END LOOP cursor_knight; CLOSE knight_cursor; /* Reset the continue handler to zero. */ SET fetched = 0; /* ---------------------------------------------------------------------- */ /* This acts as an exception handling block. */ IF duplicate_key = 1 OR foreign_key = 1 THEN /* This undoes all DML statements to this point in the procedure. */ ROLLBACK TO SAVEPOINT both_or_none; ELSE /* This commits the writes. */ COMMIT; END IF; END; $$ -- Reset the delimiter to the default. DELIMITER ; |
Here’s the better option with an embedded query:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 | -- Conditionally drop the procedure. SELECT 'UPLOAD_KINGDOM' AS "Drop Procedure"; DROP PROCEDURE IF EXISTS upload_kingdom; -- Reset the execution delimiter to create a stored program. DELIMITER $$ -- The parentheses after the procedure name must be there or the MODIFIES SQL DATA raises an compile time exception. CREATE PROCEDURE upload_kingdom() MODIFIES SQL DATA BEGIN /* Declare a handler variables. */ DECLARE duplicate_key INT DEFAULT 0; DECLARE foreign_key INT DEFAULT 0; /* Declare a duplicate key handler */ DECLARE CONTINUE HANDLER FOR 1062 SET duplicate_key = 1; DECLARE CONTINUE HANDLER FOR 1216 SET foreign_key = 1; /* ---------------------------------------------------------------------- */ /* Start transaction context. */ START TRANSACTION; /* Set savepoint. */ SAVEPOINT both_or_none; /* Using subqueries update the targets. */ INSERT INTO knight ( SELECT kn.knight_id , kki.knight_name , k.kingdom_id , kki.allegiance_start_date AS start_date , kki.allegiance_end_date AS end_date FROM kingdom_knight_import kki INNER JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population LEFT JOIN knight kn ON k.kingdom_id = kn.kingdom_allegiance_id AND kki.knight_name = kn.knight_name AND kki.allegiance_start_date = kn.allegiance_start_date AND kki.allegiance_end_date = kn.allegiance_end_date ) ON DUPLICATE KEY UPDATE knight_id = kn.knight_id; INSERT INTO knight ( SELECT kn.knight_id , kki.knight_name , k.kingdom_id , kki.allegiance_start_date AS start_date , kki.allegiance_end_date AS end_date FROM kingdom_knight_import kki INNER JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population LEFT JOIN knight kn ON k.kingdom_id = kn.kingdom_allegiance_id AND kki.knight_name = kn.knight_name AND kki.allegiance_start_date = kn.allegiance_start_date AND kki.allegiance_end_date = kn.allegiance_end_date ) ON DUPLICATE KEY UPDATE knight_id = kn.knight_id; /* ---------------------------------------------------------------------- */ /* This acts as an exception handling block. */ IF duplicate_key = 1 OR foreign_key = 1 THEN /* This undoes all DML statements to this point in the procedure. */ ROLLBACK TO SAVEPOINT both_or_none; ELSE /* This commits the writes. */ COMMIT; END IF; END; $$ -- Reset the delimiter to the default. DELIMITER ; |
Step #6 : Run the upload procedure
You can run the file by calling the stored procedure built by the script. The procedure ensures that records are inserted or updated into their respective tables.
CALL upload_kingdom; |
Step #7 : Test the results of the upload procedure
You can test whether or not it worked by running the following queries.
-- Check the kingdom table. SELECT * FROM kingdom; SELECT * FROM knight; |
It should display the following information:
+------------+--------------+------------+ | kingdom_id | kingdom_name | population | +------------+--------------+------------+ | 1 | Narnia | 77600 | | 2 | Narnia | 42100 | | 3 | Camelot | 15200 | +------------+--------------+------------+ +-----------+-------------------------+-----------------------+-----------------------+---------------------+ | knight_id | knight_name | kingdom_allegiance_id | allegiance_start_date | allegiance_end_date | +-----------+-------------------------+-----------------------+-----------------------+---------------------+ | 1 | 'Peter the Magnificent' | 1 | 1272-03-20 | 1292-06-09 | | 2 | 'Edmund the Just' | 1 | 1272-03-20 | 1292-06-09 | | 3 | 'Susan the Gentle' | 1 | 1272-03-20 | 1292-06-09 | | 4 | 'Lucy the Valiant' | 1 | 1272-03-20 | 1292-06-09 | | 5 | 'Peter the Magnificent' | 2 | 1531-04-12 | 1531-05-31 | | 6 | 'Edmund the Just' | 2 | 1531-04-12 | 1531-05-31 | | 7 | 'Susan the Gentle' | 2 | 1531-04-12 | 1531-05-31 | | 8 | 'Lucy the Valiant' | 2 | 1531-04-12 | 1531-05-31 | | 9 | 'King Arthur' | 3 | 0631-03-10 | 0686-12-12 | | 10 | 'Sir Lionel' | 3 | 0631-03-10 | 0686-12-12 | | 11 | 'Sir Bors' | 3 | 0631-03-10 | 0635-12-12 | | 12 | 'Sir Bors' | 3 | 0640-03-10 | 0686-12-12 | | 13 | 'Sir Galahad' | 3 | 0631-03-10 | 0686-12-12 | | 14 | 'Sir Gawain' | 3 | 0631-03-10 | 0686-12-12 | | 15 | 'Sir Tristram' | 3 | 0631-03-10 | 0686-12-12 | | 16 | 'Sir Percival' | 3 | 0631-03-10 | 0686-12-12 | | 17 | 'Sir Lancelot' | 3 | 0670-09-30 | 0682-12-12 | +-----------+-------------------------+-----------------------+-----------------------+---------------------+ |
You can rerun the procedure to check that it doesn’t alter any information, then you could add a new knight to test the insertion portion.
PHP Binding a Wildcard
Somebody asked me about how you bind a parameter in mysqli
when a multiple character wildcard needs to be next to the variable value. They’d found that it worked when creating a SQL statement by concatenation (string gluing), like this:
$sql = "SELECT item_title FROM item WHERE item_title LIKE '%".$title."%'"; |
However, they tried to bind the variable following the same pattern, and found that it failed. They used the following type of syntax:
$sql = "SELECT item_title FROM item WHERE item_title LIKE '%?%'"; |
It raised the following error:
Warning: mysqli_stmt_bind_param() [function.mysqli-stmt-bind-param]: Number of variables doesn't match number of parameters in prepared statement in program_name on line # |
The reason is the parser, it expects variables to be independent tokens in the SQL string. You can’t bind a string into the shell of an apostrophe delimited string. You could naturally, make an assignment to the local PHP variable before binding it to the variable, like this:
$title = '%'.$title.'%'; |
It’s actually a better idea to concatenate the multiple character wildcard operator inside the SQL statement. The correct syntax requires that you use the CONCAT()
function. You could reset to use piped concatenation but generally you should avoid that on the MySQL platform (see this post for an explanation of SQL concatenation on Oracle, MySQL, and SQL Server).
This is the required statement when using a MySQL database:
$sql = "SELECT item_title FROM item WHERE item_title LIKE CONCAT('%',?,'%')"; |
Complete Code Sample ↓
Expand this section to see the complete working code sample.
The first component for this program is an include file for the database credentials:
<?php // Connection variables. define('HOSTNAME',"localhost"); define('USERNAME',"student"); define('PASSWORD',"student"); define('DATABASE',"sampledb"); ?> |
Once you’ve placed the credentials in your directory, you can put this in the same directory and then call it from your browser. At least, you can provided you’ve created the user with the required password, and the database on the standard 3306
port.
<?php // Set database credentials. include_once("MySQLCredentials.inc"); // Return successful attempt to connect to the database. if (!$c = @mysqli_connect(HOSTNAME,USERNAME,PASSWORD,DATABASE)) { // Print user message. print "Sorry! The connection to the database failed. Please try again later."; // Assign the OCI error and format double and single quotes. print mysqli_error(); // Kill the resource. die(); } else { // Declare input variables. $title = (isset($_GET['title'])) ? $_GET['title'] : $title = "RoboCop"; query_insert($title); } // Query results afret an insert. function query_insert($title) { // Return successful attempt to connect to the database. if (!$c = @mysqli_connect(HOSTNAME,USERNAME,PASSWORD,DATABASE)) { // Print user message. print "Sorry! The connection to the database failed. Please try again later."; // Assign the OCI error and format double and single quotes. print mysqli_error(); // Kill the resource. die(); } else { // Initialize a statement in the scope of the connection. $stmt = mysqli_stmt_init($c); // Declare a SQL SELECT statement returning a CLOB. $sql = "SELECT item_title FROM item WHERE item_title LIKE CONCAT('%',?,'%')"; // Prepare statement. if (mysqli_stmt_prepare($stmt,$sql)) { mysqli_stmt_bind_param($stmt,"s",$title); // Execute it and print success or failure message. if (mysqli_stmt_execute($stmt)) { // Store result. mysqli_stmt_store_result($stmt); // Bind result to local variable. mysqli_stmt_bind_result($stmt,$item_title); // Open HTML table and print header. $out = '<table border="1" cellpadding="3" cellspacing="0">'; $out .= '<tr>'; $out .= '<th align="center" style="font-weight:bold;padding:2px;margin:1px;background:#8DB3E2;width:300px">Item Title</th>'; $out .= '</tr>'; // Read result. while (mysqli_stmt_fetch($stmt)) { $out .= '<tr>'; $out .= '<td style="padding:2px;margin:1px;background:#DBE5F1;">'.$item_title.'</td>'; $out .= '</tr>'; } // Close the HTML table. $out .= '</table>'; // Print the HTML table. print $out; } } // Free system resources. mysqli_stmt_free_result($stmt); // Disconnect from database. mysqli_close($c); } } ?> |
Decoding 1005 on MySQL
If you’re using referential integrity on MySQL, you can easily run into this error with the InnoDB engine. It frequently seems to occur with an ALTER TABLE
statement. It can mean many things but typically it means the data types don’t match between the foreign key and primary key column. Likewise, it can mean one of the column data types disagrees in a multiple-column foreign to multiple-column primary key constraint.
The error by itself isn’t very handy. This is a sample:
ERROR 1005 (HY000): Can't create table 'sampledb.#sql-4a0_2' (errno: 150) |
You can try SHOW WARNINGS
but you’ll discover more about the error by running the following command as the root
superuser:
mysql> SHOW engine innodb STATUS; |
It returns a dump of the InnoDB’s activity. You can see it by unfolding the complete log, if you’re interested in the details. The significant part of the log to solve this type of problem is:
------------------------ LATEST FOREIGN KEY ERROR ------------------------ 100130 17:16:57 Error IN FOREIGN KEY CONSTRAINT OF TABLE sampledb/#sql-4a0_2: FOREIGN KEY(member_type) REFERENCES common_lookup(common_lookup_id): Cannot find an INDEX IN the referenced TABLE WHERE the referenced COLUMNS appear AS the FIRST COLUMNS, OR COLUMN types IN the TABLE AND the referenced TABLE do NOT MATCH FOR CONSTRAINT. |
While this is more information, it isn’t necessarily enough to solve the problem when you’re new to MySQL. The first place you should look is whether the data types agree between the foreign key and primary key columns.
The most common variation that I’ve run into is where the primary key column uses a int unsigned
data type and the foreign key column uses an int
data type. It’s quite nice that the InnoDB Engine stops this cold. Naturally, you fix it by changing the foreign key data type to match the int unsigned
data type.
Complete Log File ↓
If you want to see what the complete dump of the InnoDB status is, click on the title above.
===================================== 100130 17:34:04 INNODB MONITOR OUTPUT ===================================== Per second averages calculated from the last 0 seconds ---------- SEMAPHORES ---------- OS WAIT ARRAY INFO: reservation count 11, signal count 11 Mutex spin waits 0, rounds 80, OS waits 2 RW-shared spins 18, OS waits 9; RW-excl spins 1, OS waits 0 ------------------------ LATEST FOREIGN KEY ERROR ------------------------ 100130 17:16:57 Error in foreign key constraint of table sampledb/#sql-4a0_2: FOREIGN KEY(member_type) REFERENCES common_lookup(common_lookup_id): Cannot find an index in the referenced table where the referenced columns appear as the first columns, or column types in the table and the referenced table do not match for constraint. Note that the internal storage type of ENUM and SET changed in tables created with >= InnoDB-4.1.12, and such columns in old tables cannot be referenced by such columns in new tables. See http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html for correct foreign key definition. ------------ TRANSACTIONS ------------ Trx id counter 0 12958 Purge done for trx's n:o < 0 12956 undo n:o < 0 0 History list length 7 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0 0, not started, OS thread id 3924 MySQL thread id 13, query id 800 localhost 127.0.0.1 root show engine innodb status ---TRANSACTION 0 12957, not started, OS thread id 3276 MySQL thread id 2, query id 797 localhost 127.0.0.1 student -------- FILE I/O -------- I/O thread 0 state: wait Windows aio (insert buffer thread) I/O thread 1 state: wait Windows aio (log thread) I/O thread 2 state: wait Windows aio (read thread) I/O thread 3 state: wait Windows aio (write thread) Pending normal aio reads: 0, aio writes: 0, ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0 Pending flushes (fsync) log: 0; buffer pool: 0 136 OS file reads, 1112 OS file writes, 704 OS fsyncs 0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s ------------------------------------- INSERT BUFFER AND ADAPTIVE HASH INDEX ------------------------------------- Ibuf: size 1, free list len 0, seg size 2, 0 inserts, 0 merged recs, 0 merges Hash table size 195193, node heap has 1 buffer(s) 0.00 hash searches/s, 0.00 non-hash searches/s --- LOG --- Log sequence number 0 9122680 Log flushed up to 0 9122680 Last checkpoint at 0 9122680 0 pending log writes, 0 pending chkp writes 678 log i/o's done, 0.00 log i/o's/second ---------------------- BUFFER POOL AND MEMORY ---------------------- Total memory allocated 58341818; in additional pool allocated 1481088 Dictionary memory allocated 78248 Buffer pool size 3008 Free buffers 2879 Database pages 128 Modified db pages 0 Pending reads 0 Pending writes: LRU 0, flush list 0, single page 0 Pages read 126, created 2, written 425 0.00 reads/s, 0.00 creates/s, 0.00 writes/s No buffer pool page gets since the last printout -------------- ROW OPERATIONS -------------- 0 queries inside InnoDB, 0 queries in queue 1 read views open inside InnoDB Main thread id 3408, state: waiting for server activity Number of rows inserted 454, updated 6, deleted 0, read 6026 0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s ---------------------------- END OF INNODB MONITOR OUTPUT ============================ |
The class, they survived …
The rumor is that my database is hard, but I’ve always hoped it was fun and laid the foundation of success for my students. As I walked into class to give them their final exam, they had their jackets on, which isn’t uncommon for Rexburg, Idaho in December. However, that’s normal outside but unusual in the heated classroom. Then, they all took their jackets off to show their new t-shirts.
They thought it would be fun to post on the blog, so here it is. The shirts says:
SELECT i.survived FROM michael_mclaughlin i WHERE class= CIT 320; |
It’s unfortunate that they missed the enclosing quote marks around the string literal. 😉 It should be like this:
SELECT i.survived FROM michael_mclaughlin i WHERE i.class= 'CIT 320'; |
Here’s to a great group of students who know how to read, write, and think SQL. Any openings out there for internships, please drop me a note.
Happy holidays!
Lobs with mysqli in PHP 5.3
We got away with a bit of lazy PHP, at least through PHP 5.29. Especially when we worked with LOB columns. All that is at an end with PHP 5.3. You must now call the mysqli_stmt_store_results($stmt);
function (line number 76 in the program snippet below from source) between the execute and bind result phrases.
The function transfers a result set from a prepared statement. You need to use it when performing a SELECT
, SHOW
, DESCRIBE
, or EXPLAIN
statement that you will call by the mysqli_stmt_fetch($stmt);
function.
Without using it, you may only see part of a MEDIUMTEXT
column displayed. Likewise, an image from a MEDIUMBLOB
column won’t be displayed at all unless you store their results too, by using the same function call.
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 | // Prepare statement. if (mysqli_stmt_prepare($stmt,$sql)) { mysqli_stmt_bind_param($stmt,"i",$id); // Execute it and print success or failure message. if (mysqli_stmt_execute($stmt)) { // Store result, which eliminates memory cutting off lob streams. mysqli_stmt_store_result($stmt); // Bind result to local variable. mysqli_stmt_bind_result($stmt, $desc); // Read result. mysqli_stmt_fetch($stmt); // Print result. print $desc; } } |
Also, that oldie ereg()
function is deprecated for preg_match()
in PHP 5.3 and removed in PHP 6. Don’t forget to make those changes if you were lazy like me before updating to PHP 5.3.