18.2.1 Stored Routine Syntax

A stored routine is either a procedure or a function. Stored routines are created with the CREATE PROCEDURE and CREATE FUNCTION statements (see Section 13.1.9, “CREATE PROCEDURE and CREATE FUNCTION Syntax”). A procedure is invoked using a CALL statement (see Section 13.2.1, “CALL Syntax”), and can only pass back values using output variables. A function can be called from inside a statement just like any other function (that is, by invoking the function's name), and can return a scalar value. The body of a stored routine can use compound statements (see Section 13.6, “MySQL Compound-Statement Syntax”).

Stored routines can be dropped with the DROP PROCEDURE and DROP FUNCTION statements (see Section 13.1.16, “DROP PROCEDURE and DROP FUNCTION Syntax”), and altered with the ALTER PROCEDURE and ALTER FUNCTION statements (see Section 13.1.3, “ALTER PROCEDURE Syntax”).

As of MySQL 5.0.1, a stored procedure or function is associated with a particular database. This has several implications:

(In MySQL 5.0.0, stored routines are global and not associated with a database. They inherit the default database from the caller. If a USE db_name is executed within the routine, the original default database is restored upon routine exit.)

Stored functions cannot be recursive.

Recursion in stored procedures is permitted but disabled by default. To enable recursion, set the max_sp_recursion_depth server system variable to a value greater than zero. Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup. See Section 5.1.4, “Server System Variables”, for more information.

MySQL supports a very useful extension that enables the use of regular SELECT statements (that is, without using cursors or local variables) inside a stored procedure. The result set of such a query is simply sent directly to the client. Multiple SELECT statements generate multiple result sets, so the client must use a MySQL client library that supports multiple result sets. This means the client must use a client library from a version of MySQL at least as recent as 4.1. The client should also specify the CLIENT_MULTI_RESULTS option when it connects. For C programs, this can be done with the mysql_real_connect() C API function. See Section 20.6.7.52, “mysql_real_connect(), and Section 20.6.16, “C API Support for Multiple Statement Execution”.