A stored routine is either a procedure or a function. Stored
routines are created with the
FUNCTION statements (see
Section 13.1.17, “CREATE PROCEDURE and CREATE FUNCTION Statements”). A procedure is invoked using
CALL statement (see
Section 13.2.1, “CALL Statement”), 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, “Compound Statement Syntax”).
Stored routines can be dropped with the
FUNCTION statements (see
Section 13.1.29, “DROP PROCEDURE and DROP FUNCTION Statements”), and altered with the
ALTER PROCEDURE and
ALTER FUNCTION statements (see
Section 13.1.7, “ALTER PROCEDURE Statement”).
A stored procedure or function is associated with a particular database. This has several implications:
When the routine is invoked, an implicit
USEis performed (and undone when the routine terminates).
USEstatements within stored routines are not permitted.
You can qualify routine names with the database name. This can be used to refer to a routine that is not in the current database. For example, to invoke a stored procedure
fthat is associated with the
testdatabase, you can say
When a database is dropped, all stored routines associated with it are dropped as well.
Stored functions cannot be recursive.
Recursion in stored procedures is permitted but disabled by
default. To enable recursion, set the
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 5.1.8, “Server System Variables”, for more
MySQL supports a very useful extension that enables the use of
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
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 mysql_real_connect(), and
Multiple Statement Execution Support.
A user variable referenced by a statement in a stored procedure has its type determined the first time the procedure is invoked, and retains this type each time the procedure is invoked thereafter.