Archive for the ‘sql’ Category
PostgreSQL and LPAD
While porting my Oracle code to PostgreSQL I encountered a little quirk. It’s probably not a quirk except for the fact that I’ve worked in Oracle so long. Oracle implicitly type casts so well that we seldom notice.
PostreSQL doesn’t work like Oracle. It does type cast sometimes but not very often. I tried porting the following segment from my Oracle stored procedure to PostgreSQL:
/* Add account number with zeros. */ FOR j IN 1..50 LOOP INSERT INTO account_list VALUES ( lv_airport_code||'-'||LPAD(j,6,'0') , NULL , NULL , 1002 , 1002 ); END LOOP; |
Oracle implicitly casts the integer j to a text string before running the LPAD function. In PostgreSQL, it doesn’t cast the integer to a text string before calling the LPAD function. Since the LPAD function is not overloaded, calling it with an integer, integer, and text set of parameters fails at runtime but the code doesn’t raise an exception when compiling it as a stored procedure.
At runtime, it raises the following error:
ERROR: invalid INPUT syntax FOR INTEGER: "function lpad(integer, integer, unknown) does not exist" CONTEXT: PL/pgSQL FUNCTION seed_account_list() line 48 at assignment |
You can fix the code by explicitly casting the LPAD function’s first parameter to a text string. You do that as follows below:
/* Add account number with zeros. */ FOR j IN 1..50 LOOP INSERT INTO account_list VALUES ( lv_airport_code||'-'||LPAD(j::text,6,'0') , NULL , NULL , 1002 , 1002 ); END LOOP; |
This fixes the casting problem. As always, I hope this helps those looking for a solution.
Postgres Foreign Keys
Just sorting out how to query the information_schema to discover the magic for a query of a table’s foreign key constraints. This query works to return the foreign key constraints:
SELECT conrelid::regclass::text AS table_from , conname AS foreign_key , pg_get_constraintdef(oid) FROM pg_constraint WHERE contype = 'f' AND connamespace = 'public'::regnamespace AND conrelid::regclass::text = 'rental_item' ORDER BY conrelid::regclass::text , conname; |
It returns the following for the rental_item table:
table_from | foreign_key | pg_get_constraintdef -------------+------------------+--------------------------------------------------------------------------- rental_item | fk_rental_item_1 | FOREIGN KEY (rental_id) REFERENCES rental(rental_id) rental_item | fk_rental_item_2 | FOREIGN KEY (item_id) REFERENCES item(item_id) rental_item | fk_rental_item_3 | FOREIGN KEY (created_by) REFERENCES system_user(system_user_id) rental_item | fk_rental_item_4 | FOREIGN KEY (last_updated_by) REFERENCES system_user(system_user_id) rental_item | fk_rental_item_5 | FOREIGN KEY (rental_item_type) REFERENCES common_lookup(common_lookup_id) (5 rows) |
As always, I post these tidbit for others to discover and use with less pain.
Postgres Overloaded Routines
Earlier I showed how to write an anonymous block in PostgreSQL PL/pgSQL to drop routines, like functions and procedures. However, it would only work when they’re not overloaded functions or procedures. The following lets you drop all routines, including overloaded functions and procedures. Overloaded procedures are those that share the same name but have different parameter lists.
Before you can test the anonymous block, you need to create a set of overloaded functions or procedures. You can create a set of overloaded hello procedures with the following syntax:
CREATE FUNCTION hello() RETURNS text AS $$ DECLARE output VARCHAR; BEGIN SELECT 'Hello World!' INTO output; RETURN output; END $$ LANGUAGE plpgsql; CREATE FUNCTION hello(whom text) RETURNS text AS $$ DECLARE output VARCHAR; BEGIN SELECT CONCAT('Hello ',whom,'!') INTO output; RETURN output; END $$ LANGUAGE plpgsql; CREATE FUNCTION hello(id int, whom text) RETURNS text AS $$ DECLARE output VARCHAR; BEGIN SELECT CONCAT('[',id,'] Hello ',whom,'!') INTO output; RETURN output; END $$ LANGUAGE plpgsql; |
You can test the overloaded hello function, like so from the videodb schema:
videodb=> SELECT hello(); hello -------------- Hello World! (1 ROW) videodb=> SELECT hello('Captain Marvel'); hello ----------------------- Hello Captain Marvel! (1 ROW) videodb=> SELECT hello(1,'Captain America'); hello ---------------------------- [1] Hello Captain America! (1 ROW) |
Then, you can query the information_schema to verify that you’ve created a set of overloaded procedures with the following query:
SELECT proc.specific_schema AS procedure_schema , proc.specific_name , proc.routine_name AS procedure_name , proc.external_language , args.parameter_name , args.parameter_mode , args.data_type FROM information_schema.routines proc left join information_schema.parameters args ON proc.specific_schema = args.specific_schema AND proc.specific_name = args.specific_name WHERE proc.routine_schema NOT IN ('pg_catalog', 'information_schema') AND proc.routine_type IN ('FUNCTION','PROCEDURE') ORDER BY procedure_schema , specific_name , procedure_name , args.ordinal_position; |
It should return the following:
procedure_schema | specific_name | procedure_name | external_language | parameter_name | parameter_mode | data_type ------------------+---------------+----------------+-------------------+----------------+----------------+----------- public | hello_35451 | hello | PLPGSQL | | | public | hello_35452 | hello | PLPGSQL | whom | IN | text public | hello_35453 | hello | PLPGSQL | id | IN | integer public | hello_35453 | hello | PLPGSQL | whom | IN | text (4 rows) |
The set session command maps the videodb catalog for the following anonymous block program.
SET SESSION "videodb.catalog_name" = 'videodb'; |
The following anonymous block lets you get rid of any ordinary or overloaded function and procedure:
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 | DO $$ DECLARE /* Declare an indefinite length string for SQL statement. */ sql VARCHAR; /* Declare variables to manage cursor return values. */ row RECORD; arg VARCHAR; /* Declare parameter list. */ list VARCHAR; /* Declare a routine cursor. */ routine_cursor CURSOR FOR SELECT routine_name , specific_name , routine_type FROM information_schema.routines WHERE specific_catalog = current_setting('videodb.catalog_name') AND routine_schema = 'public'; /* Declare a parameter cursor. */ parameter_cursor CURSOR (cv_specific_name varchar) FOR SELECT args.data_type FROM information_schema.parameters args WHERE args.specific_schema = 'public' AND args.specific_name = cv_specific_name; BEGIN /* Open the cursor. */ OPEN routine_cursor; <<row_loop>> LOOP /* Fetch table names. */ FETCH routine_cursor INTO row; /* Exit when no more records are found. */ EXIT row_loop WHEN NOT FOUND; /* Initialize parameter list. */ list := '('; /* Open the parameter cursor. */ OPEN parameter_cursor(row.specific_name::varchar); <<parameter_loop>> LOOP FETCH parameter_cursor INTO arg; /* Exit the parameter loop. */ EXIT parameter_loop WHEN NOT FOUND; /* Add parameter and delimit more than one parameter with a comma. */ IF LENGTH(list) > 1 THEN list := CONCAT(list,',',arg); ELSE list := CONCAT(list,arg); END IF; END LOOP; /* Close the parameter list. */ list := CONCAT(list,')'); /* Close the parameter cursor. */ CLOSE parameter_cursor; /* Concatenate together a DDL to drop the table with prejudice. */ sql := 'DROP '||row.routine_type||' IF EXISTS '||row.routine_name||list; /* Execute the DDL statement. */ EXECUTE sql; END LOOP; /* Close the routine_cursor. */ CLOSE routine_cursor; END; $$; |
Now, you possess the magic to automate cleaning up your schema when you combine this with my earlier post on dynamically dropping tables, sequences, and triggers.
DBeaver for PostgreSQL
I’m migrating my database classes from the Oracle database to the PostgreSQL database. Using the Oracle Express Edition has always required a virtualized image because students use Windows and Mac OS. Also, the university doesn’t like my use of a virtualized image. Virtualization imposes incremental cost on students to have high end laptops.
The available Docker images don’t typically support the Oracle Express Edition. That means there are licensing implications tied to Oracle.
As a committee, we decided to use PostgreSQL as our new database platform. We opted to use PostgreSQL over MySQL because it supports arrays and stored procedures. PostgreSQL PL/pgSQL are a solid implementation of stored procedures. While MySQL supports a stored procedure language, it effectively discourages using it.
We use sqlplus, mysql, or psql Command-Line Interface (CLI) as the primary interface. That’s because CLI supports production-oriented deployment and DevOps practices. The Open Source DBeaver project provides a clean native GUI management console for PostgreSQL. DBeaver also supports other databases, which pgAdmin doesn’t.
You click the Download button from the DBeaver home page. It support Windows, Mac OS, and Linux distributions. I downloaded the Linux RPM package to my Downloads directory. You can run the RPM with the following syntax:
rpm -ivh dbeaver-ce-6.2.3-stable.x86_64.rpm |
It doesn’t check for dependencies. The installation is quick. You can verify that it’s installed with the following command:
which dbeaver |
It installs here:
/usr/bin/dbeaver |
If you failed to install the PostgreSQL JAR file, you’ll see the following error message:
It will look like the following on Linux:
You should have a user in the pg_hba.conf file, like the following:
host all all 127.0.0.1/32 trust |
As always, I hope this helps those solving problems.
Java and Postgres
I wanted to get Java working with PostgreSQL to test some GUI interfaces on Linux. Figuring out the necessary JAR file for the JDBC was my first hurdle. I found it was postgreSQL-42-2.5.jar file.
You can download it with the following command line:
wget https://jdbc.postgresql.org/download/postgresql-42.2.5.jar |
I downloaded it to a Java directory off the home/student directory. Then, I added the following CLASSPATH to local java.env environment file.
#!/usr/bin/bash # Set the Java CLASSPATH environment variable to include the JDBC jar file. export set CLASSPATH=/home/student/Java/postgresql-42.2.5.jar:. |
I sourced the postgresql-42.2.5.jar file and I wrote the following JavaTest.java program:
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 | /* Import classes. */ import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement; import java.util.logging.Level; import java.util.logging.Logger; public class JavaTest { public static void main(String[] args) { /* Set default strings for the connection. */ String url = "jdbc:postgresql://localhost:5432/videodb"; String user = "student"; String password = "student"; /* Try the connection and statement. */ try { Connection conn = DriverManager.getConnection(url, user, password); Statement st = conn.createStatement(); ResultSet rs = st.executeQuery("SELECT VERSION()"); if (rs.next()) { System.out.println(rs.getString(1)); } /* Close the result set and statement. */ rs.close(); st.close(); } catch (SQLException e) { Logger logger = Logger.getLogger(JavaTest.class.getName()); logger.log(Level.SEVERE, e.getMessage(), e); } } } |
I compiled the JavaTest.java program and tested it. It failed with the following error:
FATAL: Ident authentication failed for user - Unable to connect to PostgreSQL |
The failure occurred because I hadn’t allowed the connection in PostgreSQL’s pg_hba.conf file. I changed the following line in my pg_hba.conf file:
host all all 127.0.0.1/32 ident |
to
host all all 127.0.0.1/32 trust |
Then, I restarted the postgresql-11 service, like this:
systemctl restart postgresql-11.service |
The JavaTest program ran successfully and returned:
PostgreSQL 11.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 9.1.1 20190503 (Red Hat 9.1.1-1), 64-bit |
You can extend the logic to output a comma-separated value file by leveraging the JDBC help page, like this:
/* Import classes. */ import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.ResultSetMetaData; import java.sql.SQLException; import java.sql.Statement; import java.util.logging.Level; import java.util.logging.Logger; /* Create class withonly a static method for runtime testing. */ public class JavaReturnSet { public static void main(String[] args) { /* Set default strings for the connection. */ String url = "jdbc:postgresql://localhost:5432/videodb"; String user = "student"; String password = "student"; /* Try the connection and statement. */ try { /* Set connection, statement, and result set. */ Connection conn = DriverManager.getConnection(url, user, password); Statement st = conn.createStatement(); /* Use the + to concatenate lines for query clarity. */ ResultSet rs = st.executeQuery("SELECT m.account_number\n" + ", CONCAT(c.last_name,', ',c.first_name)\n" + "FROM member m JOIN contact c\n" + "ON m.member_id = c.member_id"); /* Get query metadata for subsequent management of results. */ ResultSetMetaData rsmd = rs.getMetaData(); String line; while (rs.next()) { /* Initialize the line output for each row. */ line = ""; /* Process the columns. */ for (int i = 0; i < rsmd.getColumnCount(); i++) { if (rsmd.getColumnType(i + 1) == 12) { line = line + "\"" + rs.getString(i + 1) + "\""; } else { line = line + rs.getInt(i + 1); } /* Put a comma between output columns. */ if (i < rsmd.getColumnCount() - 1) { line = line + ","; } } System.out.println(line); } /* Close the result set and statement. */ rs.close(); st.close(); } catch (SQLException e) { Logger logger = Logger.getLogger(JavaReturnSet.class.getName()); logger.log(Level.SEVERE, e.getMessage(), e); } } } |
As always, I hope this helps those looking for a clean solution.
Postgres Drop Structures
While building my PostgreSQL environment for the class, I had to write a couple utilities. They do the following:
- Drops all the tables from a schema.
- Drops all the sequences from a schema that aren’t tied to an
_idcolumn with aSERIALdata type. - Drops all the functions and procedures (qualified as routines) from a schema.
- Drops all the triggers from a schema.
The following gives you the code for all four files: drop_tables.sql, drop_sequences.sql, drop_routines.sql, and drop_triggers.sql.
- The drop_tables.sql Script:
- The drop_sequences.sql script:
- The drop_routines.sql script:
- The drop_triggers.sql script:
/* Verify all tables present. */ SELECT table_name FROM information_schema.tables WHERE table_catalog = current_setting('videodb.catalog_name') AND table_schema = 'public'; DO $$ DECLARE /* Declare an indefinite length string and record variable. */ sql VARCHAR; row RECORD; /* Declare a cursor. */ table_cursor CURSOR FOR SELECT table_name FROM information_schema.tables WHERE table_catalog = current_setting('videodb.catalog_name') AND table_schema = 'public'; BEGIN /* Open the cursor. */ OPEN table_cursor; LOOP /* Fetch table names. */ FETCH table_cursor INTO row; /* Exit when no more records are found. */ EXIT WHEN NOT FOUND; /* Concatenate together a DDL to drop the table with prejudice. */ sql := 'DROP TABLE IF EXISTS '||row.table_name||' CASCADE'; /* Execute the DDL statement. */ EXECUTE sql; END LOOP; /* Close the cursor. */ CLOSE table_cursor; END; $$; /* Verify all tables are dropped. */ SELECT table_name FROM information_schema.tables WHERE table_catalog = current_setting('videodb.catalog_name') AND table_schema = 'public'; |
/* Verify all tables present. */ SELECT sequence_name FROM information_schema.sequences WHERE sequence_catalog = current_setting('videodb.catalog_name') AND sequence_schema = 'public'; DO $$ DECLARE /* Declare an indefinite length string and record variable. */ sql VARCHAR; row RECORD; /* Declare a cursor. */ sequence_cursor CURSOR FOR SELECT sequence_name FROM information_schema.sequences WHERE sequence_catalog = current_setting('videodb.catalog_name') AND sequence_schema = 'public'; BEGIN /* Open the cursor. */ OPEN sequence_cursor; LOOP /* Fetch table names. */ FETCH sequence_cursor INTO row; /* Exit when no more records are found. */ EXIT WHEN NOT FOUND; /* Concatenate together a DDL to drop the table with prejudice. */ sql := 'DROP SEQUENCE IF EXISTS '||row.sequence_name; /* Execute the DDL statement. */ EXECUTE sql; END LOOP; /* Close the cursor. */ CLOSE sequence_cursor; END; $$; /* Verify all tables are dropped. */ SELECT sequence_name FROM information_schema.sequences WHERE sequence_catalog = current_setting('videodb.catalog_name') AND sequence_schema = 'public'; |
/* Verify all tables present. */ SELECT routine_name , routine_type FROM information_schema.routines WHERE specific_catalog = current_setting('videodb.catalog_name') AND specific_schema = 'public'; DO $$ DECLARE /* Declare an indefinite length string and record variable. */ sql VARCHAR; row RECORD; /* Declare a cursor. */ routine_cursor CURSOR FOR SELECT routine_name , routine_type FROM information_schema.routines WHERE specific_catalog = current_setting('videodb.catalog_name') AND routine_schema = 'public'; BEGIN /* Open the cursor. */ OPEN routine_cursor; LOOP /* Fetch table names. */ FETCH routine_cursor INTO row; /* Exit when no more records are found. */ EXIT WHEN NOT FOUND; /* Concatenate together a DDL to drop the table with prejudice. */ sql := 'DROP '||row.routine_type||' IF EXISTS '||row.routine_name; /* Execute the DDL statement. */ EXECUTE sql; END LOOP; /* Close the cursor. */ CLOSE routine_cursor; END; $$; /* Verify all tables are dropped. */ SELECT routine_name , routine_type FROM information_schema.routines WHERE specific_catalog = 'videodb' AND specific_schema = 'public'; |
/* Verify all tables present. */ SELECT trigger_name FROM information_schema.triggers WHERE trigger_catalog = current_setting('videodb.catalog_name') AND trigger_schema = 'public'; DO $$ DECLARE /* Declare an indefinite length string and record variable. */ sql VARCHAR; row RECORD; /* Declare a cursor. */ trigger_cursor CURSOR FOR SELECT trigger_name FROM information_schema.triggers WHERE trigger_catalog = current_setting('videodb.catalog_name') AND trigger_schema = 'public'; BEGIN /* Open the cursor. */ OPEN trigger_cursor; LOOP /* Fetch table names. */ FETCH trigger_cursor INTO row; /* Exit when no more records are found. */ EXIT WHEN NOT FOUND; /* Concatenate together a DDL to drop the table with prejudice. */ sql := 'DROP TRIGGER IF EXISTS '||row.trigger_name; /* Execute the DDL statement. */ EXECUTE sql; END LOOP; /* Close the cursor. */ CLOSE trigger_cursor; END; $$; /* Verify all tables are dropped. */ SELECT trigger_name FROM information_schema.triggers WHERE trigger_catalog = current_setting('videodb.catalog_name') AND trigger_schema = 'public'; |
You can create a cleanup_catalog.sql script to call all four in sequence, like the following:
\i /home/student/Data/cit225/postgres/lib/utility/drop_tables.sql \i /home/student/Data/cit225/postgres/lib/utility/drop_sequences.sql \i /home/student/Data/cit225/postgres/lib/utility/drop_routines.sql \i /home/student/Data/cit225/postgres/lib/utility/drop_triggers.sql |
The nice thing about this approach is that you won’t see any notices when tables, sequences, routines, or triggers aren’t found. It’s a clean approach to cleaning the schema for a testing environment.
Postgres SQL Nuance
I ran across an interesting nuance between Oracle and Postgres with the double-pipe operator. I found that the following query failed to cross port from Oracle to Postgres:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | COL account_number FORMAT A10 HEADING "Account|Number" COL full_name FORMAT A16 HEADING "Name|(Last, First MI)" COL city FORMAT A12 HEADING "City" COL state_province FORMAT A10 HEADING "State" COL telephone FORMAT A18 HEADING "Telephone" SELECT m.account_number , c.last_name || ', ' || c.first_name || CASE WHEN c.middle_name IS NOT NULL THEN ' ' || c.middle_name END AS full_name , a.city , a.state_province , t.country_code || '-(' || t.area_code || ') ' || t.telephone_number AS telephone FROM member m INNER JOIN contact c ON m.member_id = c.member_id INNER JOIN address a ON c.contact_id = a.contact_id INNER JOIN street_address sa ON a.address_id = sa.address_id INNER JOIN telephone t ON c.contact_id = t.contact_id AND a.address_id = t.address_id WHERE c.last_name = 'Winn'; |
In Oracle, a CASE statement ignores the null of a missing ELSE clause between lines 4 and 5. Oracle assumes a null value is an empty string when concatenated to a string with the double-piped concatenation operator. Oracle’s implementation differs from the ANSI standard and is non-compliant.
It would display the following thanks to the SQL reporting features that don’t exist in other Command-Line Interface (CLI) implementations, like mysql, psql, sqlcmd, or cql:
Account Name Number (Last, First MI) City State Telephone ---------- ---------------- ------------ ---------- ------------------ B293-71445 Winn, Randi San Jose CA 001-(408) 111-1111 B293-71445 Winn, Brian San Jose CA 001-(408) 111-1111 |
However, it fails in Postgres without a notice, warning, or error. Postgres simply returns a null string for the missing ELSE clause and follows the rule that any string concatenated against a null is a null. That means it retunes a null value for the full_name column above. The Postgres behavior is the ANSI standard behavior. After years of working with Oracle it was interesting to have this pointed out while porting a query.
You can fix the statement in Postgres by adding an explicit ELSE clause on a new line 5 that appends an empty string, like this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | SELECT m.account_number , c.last_name || ', ' || c.first_name || CASE WHEN c.middle_name IS NOT NULL THEN ' ' || c.middle_name ELSE '' END AS full_name , a.city , a.state_province , t.country_code || '-(' || t.area_code || ') ' || t.telephone_number AS telephone FROM member m INNER JOIN contact c ON m.member_id = c.member_id INNER JOIN address a ON c.contact_id = a.contact_id INNER JOIN street_address sa ON a.address_id = sa.address_id INNER JOIN telephone t ON c.contact_id = t.contact_id AND a.address_id = t.address_id WHERE c.last_name = 'Winn'; |
It would display:
account_number | full_name | city | state_province | telephone ----------------+-------------+----------+----------------+-------------------- B293-71445 | Winn, Randi | San Jose | CA | 001-(408) 111-1111 B293-71445 | Winn, Brian | San Jose | CA | 001-(408) 111-1111 (2 rows) |
As always, I hope this helps those looking to solve a problem.
Postgres Foreign Constraints
You can’t disable a foreign key constraint in Postgres, like you can do in Oracle. However, you can remove the foreign key constraint from a column and then re-add it to the column.
Here’s a quick test case in five steps:
- Drop the
bigandlittletable if they exists. The firstdropstatement requires a cascade because there is a dependentlittletable that holds a foreign key constraint against the primary key column of thebigtable. The second drop statement does not require the cascade keyword because there is not a dependent foreign key constraint.DROP TABLE IF EXISTS big CASCADE; DROP TABLE IF EXISTS little;
- Create the
bigandlittletables:-- Create the big table. CREATE TABLE big ( big_id SERIAL , big_text VARCHAR(20) NOT NULL , CONSTRAINT pk_little_1 PRIMARY KEY (big_id)); -- Display the big table. \d big -- Create little table. CREATE TABLE little ( little_id SERIAL , big_id INTEGER NOT NULL , little_text VARCHAR(20) NOT NULL , CONSTRAINT fk_little_1 FOREIGN KEY (big_id) REFERENCES big (big_id)); -- Display the little table. \d little
If you failed to designate the
big_idcolumn as a primary key constrained, Postgres will raise the following exception:ERROR: there IS no UNIQUE CONSTRAINT matching given KEYS FOR referenced TABLE "big"
- Insert a non-compliant row in the
littletable. An insert statement into thelittletable with a value for the foreign key column that does not exist in thebig_idcolumn of thebigtable would fail with the following error:ERROR: INSERT OR UPDATE ON TABLE "little" violates FOREIGN KEY CONSTRAINT "fk_little_1" DETAIL: KEY (big_id)=(2) IS NOT present IN TABLE "big".
Re-enabling the foreign key constraint, the insert statement succeeds after you first insert a new row into the
bigtable with the foreign key value for thelittletable as its primary key. The following two insert statements add a row to both thebigandlittletable:-- Insert into a big table. INSERT INTO big (big_text) VALUES ('Cat in the Hat 2'); -- Insert into a little table. INSERT INTO little (big_id ,little_text) VALUES ( 2 ,'Thing 3');
Then, you can query it like this:
SELECT * FROM big b JOIN little l ON b.big_id = l.big_id;
big_id | big_text | little_id | big_id | little_text --------+------------------+-----------+--------+------------- 1 | Cat IN the Hat 1 | 1 | 1 | Thing 1 1 | Cat IN the Hat 1 | 2 | 1 | Thing 2 2 | Cat IN the Hat 2 | 3 | 2 | Thing 3 (3 ROWS)
- You can drop a foreign key constraint with the following syntax:
ALTER TABLE little DROP CONSTRAINT fk_little_1;
- You can add a foreign key constraint with the following syntax:
ALTER TABLE little ADD CONSTRAINT fk_little_1 FOREIGN KEY (big_id) REFERENCES big (big_id);
As always, I hope this helps you solve problems.
Postgres Remove Constraints
You can’t disable a not null constraint in Postgres, like you can do in Oracle. However, you can remove the not null constraint from a column and then re-add it to the column.
Here’s a quick test case in four steps:
- Drop a demo table if it exists:
DROP TABLE IF EXISTS demo;
- Create a
demotable if it exists:CREATE TABLE demo ( demo_id SERIAL , demo_text VARCHAR(20) NOT NULL );
- Insert a compliant row in the
demotable if it exists:INSERT INTO demo (demo_text) VALUES ('Thing 1');
Attempt to insert another row with a null value in the
demo_textcolumn:INSERT INTO demo (demo_text) VALUES (NULL);
It raises the following error:
INSERT 0 1 psql:remove_not_null.sql:22: ERROR: NULL VALUE IN COLUMN "demo_text" violates not-NULL CONSTRAINT DETAIL: Failing ROW contains (2, NULL).
- You can drop the not null constraint from the
demo_textcolumn:ALTER TABLE demo ALTER COLUMN demo_text DROP NOT NULL;
You can now successfully insert a row with a
demo_textcolumn value of null. After you have performed your table maintenance you can add the not null constraint back on to thedemo_textcolumn.You need to update the row with a null value in the
demo_textcolumn with a valid value before you re-add the not null constraint. The following shows an update statement that replaces the null value with a text string:UPDATE demo SET demo_text = 'Thing 2' WHERE demo_text IS NULL;
Now, you can change the
demo_textcolumn back to a not null constrained column with the following syntax.ALTER TABLE demo ALTER COLUMN demo_text SET NOT NULL;
While you can not defer the constraint, removing it and adding it back works well.
Postgres Check Constraints
The Postgres 11 database documentation says that it supports naming constraints. While you can create a table with named constraints inside the CREATE TABLE statement, the names are not assigned to the not null check constraint.
Here’s a quick test case in three steps:
- Drop a demo table if it exists:
DROP TABLE IF EXISTS demo;
- Drop a demo table if it exists:
CREATE TABLE demo ( demo_id SERIAL , demo_text VARCHAR(20) CONSTRAINT nn_demo_1 NOT NULL );
- Create a
demotable if it exists:SELECT substr(check_clause,1,strpos(check_clause,' ')-1) AS check_column , constraint_name FROM information_schema.check_constraints WHERE check_clause LIKE 'demo_text%';
You should see the following output with a parsed
check_columnname and the system generatedcheckconstraint name rather than thenn_demo_1constraint name:check_column | constraint_name --------------+----------------------- demo_text | 2200_18896_2_not_null (1 row)
On the bright side, you can name primary key and foreign key constraints.
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