RETURN expr The RETURN statement terminates execution of a stored function and
returns the value expr to the function caller. There must be at least
one RETURN statement in a stored function. If the function has multiple exit points, all exit points must have a RETURN.
This statement is not used in stored procedures, triggers, or events. LEAVE can be used instead.
The following example shows that RETURN can return the result of a scalar subquery:
CREATE FUNCTION users_count() RETURNS BOOL
READS SQL DATA
BEGIN
RETURN (SELECT COUNT(DISTINCT User) FROM mysql.user);
END;