mysql清除表中的重複數據

mysql清除表中的重複數據

例子：我這是用戶角色關聯表，根據用戶id和角色id確認唯一一條數據

1：先查出重複數據，可以根據自己的需求更換sql中的字段，或者增加條件

SELECT
	a.ROLE_ID,
	a.USER_ACCOUNT_ID 
FROM
	cof_role_user a 
WHERE
	( a.ROLE_ID, a.USER_ACCOUNT_ID ) IN ( SELECT role_id, USER_ACCOUNT_ID FROM cof_role_user GROUP BY role_id, USER_ACCOUNT_ID HAVING count( * ) > 1 ) 
ORDER BY
	USER_ACCOUNT_ID

2：刪除重複數據並保留一條數據，id是主鍵，必須保留一條數據

delete from cof_role_user   
where (role_id,USER_ACCOUNT_ID) 
in  (SELECT * from 
(select role_id,USER_ACCOUNT_ID from cof_role_user  group by role_id,USER_ACCOUNT_ID having count(*) > 1)
 a) 
and id 
not in (SELECT * from
(select min(id) from cof_role_user  group by role_id,USER_ACCOUNT_ID having count(*)>1) b
)