DROP TABLE IF EXISTS t2;

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 ( id serial, f int, msg varchar(100), index(f));

CREATE TABLE t2 ( id serial, f int, unique(f), constraint `f` foreign key (f) references t1 (f) ON UPDATE CASCADE);

insert into t1 values(1, 23, 'abc');

insert into t1 values(2, 42, 'xyz');

INSERT INTO t2 VALUES(1, 23);

INSERT INTO t2 VALUES(2, 42);

DROP PROCEDURE IF EXISTS p1;

DELIMITER //

CREATE PROCEDURE p1()

BEGIN

  DECLARE exit HANDLER FOR  SQLEXCEPTION

  BEGIN

    SELECT 42;

  END; 

  UPDATE t1 SET f=23 WHERE f=42;

END;

//

DELIMITER ;

CALL p1();

+----+

| 42 |

+----+

| 42 |

+----+

1 row in set (0.00 sec)

ERROR 1761 (23000): Foreign key constraint for table 't1', record '2' would lead to a duplicate entry in table 't2', key 'f'