A Stored Function is a defined function that is called from within an SQL statement like a regular function, and returns a single value.

Creating Stored Functions

Here's a skeleton example to see a stored function in action:

DELIMITER //

CREATE FUNCTION FortyTwo() RETURNS TINYINT DETERMINISTIC
BEGIN
 DECLARE x TINYINT;
 SET x = 42;
 RETURN x;
END 

//

DELIMITER ;

First, the delimiter is changed, since the function definition will contain the regular semicolon delimiter. See Delimiters in the mysql client for more. Then the function is named FortyTwo and defined to return a tinyin. The DETERMINISTIC keyword is not necessary in all cases (although if binary logging is on, leaving it out will throw an error), and is to help the query optimizer choose a query plan. A deterministic function is one that, given the same arguments, will always return the same result.

Next, the function body is placed between BEGIN and END statements. It declares a tinyint, X, which is simply set to 42, and this is the result returned.

SELECT FortyTwo();
+------------+
| FortyTwo() |
+------------+
|         42 |
+------------+

Of course, a function that doesn't take any arguments is of little use. Here's a more complex example:

DELIMITER //
CREATE FUNCTION VatCents(price DECIMAL(10,2)) RETURNS INT DETERMINISTIC
BEGIN
 DECLARE x INT;
 SET x = price * 114;
 RETURN x;
END //
Query OK, 0 rows affected (0.04 sec)
DELIMITER ;

This function takes an argument, price which is defined as a DECIMAL, and returns an INT.

Take a look at the CREATE FUNCTION page for more details.

From MariaDB 10.3.3, it is also possible to create stored aggregate functions.

Stored Function listings and definitions

To find which stored functions are running on the server, use SHOW FUNCTION STATUS.

SHOW FUNCTION STATUS\G
*************************** 1. row ***************************
                  Db: test
                Name: VatCents
                Type: FUNCTION
             Definer: root@localhost
            Modified: 2013-06-01 12:40:31
             Created: 2013-06-01 12:40:31
       Security_type: DEFINER
             Comment: 
character_set_client: utf8
collation_connection: utf8_general_ci
  Database Collation: latin1_swedish_ci
1 row in set (0.00 sec)

or query the routines table in the INFORMATION_SCHEMA database directly:

SELECT ROUTINE_NAME FROM INFORMATION_SCHEMA.ROUTINES WHERE
  ROUTINE_TYPE='FUNCTION';
+--------------+
| ROUTINE_NAME |
+--------------+
| VatCents     |
+--------------+

To find out what the stored function does, use SHOW CREATE FUNCTION.

SHOW CREATE FUNCTION VatCents\G
*************************** 1. row ***************************
            Function: VatCents
            sql_mode: 
     Create Function: CREATE DEFINER=`root`@`localhost` FUNCTION `VatCents`(price DECIMAL(10,2)) RETURNS int(11)
    DETERMINISTIC
BEGIN
 DECLARE x INT;
 SET x = price * 114;
 RETURN x;
END
character_set_client: utf8
collation_connection: utf8_general_ci
  Database Collation: latin1_swedish_ci

Dropping and Updating Stored Functions

To drop a stored function, use the DROP FUNCTION statement.

DROP FUNCTION FortyTwo;

To change the characteristics of a stored function, use ALTER FUNCTION. Note that you cannot change the parameters or body of a stored function using this statement; to make such changes, you must drop and re-create the function using DROP FUNCTION and CREATE FUNCTION.

Permissions in Stored Functions

See the article Stored Routine Privileges.

See Also

• CREATE FUNCTION
• SHOW CREATE FUNCTION
• DROP FUNCTION
• Stored Routine Privileges
• SHOW FUNCTION STATUS
• Information Schema ROUTINES Table
• Stored Aggregate Functions.