DELETE FROM a WHERE a_id IN
(SELECT a_id FROM
(SELECT a.a_id FROM a LEFT JOIN b ON a.a_id = b.a_id WHERE b.a_id IS NULL) AS tmp
);
(SELECT a_id FROM
(SELECT a.a_id FROM a LEFT JOIN b ON a.a_id = b.a_id WHERE b.a_id IS NULL) AS tmp
);
自己试试吧
where s=del_value and
s not in (select s from b where s=del_value );