因為有遇到這個問題 所以特別寫出來備忘 SELECT * FROM bb_downloads a WHERE (SELECT count( * ) FROM bb_downloads b WHERE b.post_id = a.post_id AND b.url = a.url ) >1 ORDER BY `post_id`,`download_times` DESC reference UPDATE: 2007/10/14 找出不相同的資料 非常的easy select r.user_id from rtable r WHERE r.user_id not in (select a.user_id from atable a) 刪除不相同的資料 delete from rtable WHERE user_id not [...]