Syntax:
RELEASE_LOCK(str)

Releases the lock named by the string str that was obtained with
GET_LOCK(). Returns 1 if the lock was released, 0 if the lock was not
established by this thread (in which case the lock is not released),
and NULL if the named lock did not exist. The lock does not exist if it
was never obtained by a call to GET_LOCK() or if it has previously been
released.

The DO statement is convenient to use with RELEASE_LOCK(). See [HELP
DO].

URL: http://dev.mysql.com/doc/refman/5.1/en/miscellaneous-functions.html

