1、查找重复数据：SELECT * FROM `table_a` as a
WHERE NOT EXISTS (SELECT * FROM table_b as b WHERE b.a_id=a.id)) as c
GROUP BY c.title HAVING count(1) > 1；2、查找重复数据中需要保留的数据：SELECT MIN(c.id) as id FROM
(SELECT * FROM `table_a` as a WHERE NOT EXISTS (SELECT * FROM table_b&n...