CREATE TABLE t1 ( a INT, b CHAR(3) );
INSERT INTO t1 VALUES ( 1, 'foo' );
CREATE TABLE t2 ( c CHAR(3), d INT );
INSERT INTO t2 VALUES ( 'foo', 1 );
UPDATE IGNORE t1, t2 SET b = 'bar', c = 'bar'
  WHERE a != ( SELECT 1 UNION SELECT 2 );
Warnings:
Warning	1242	Subquery returns more than 1 row
DROP TABLE t1, t2;