MySQL查询重复数据

MySQL查询重复数据

 

假设有一个用户表 user，数据如下：

1、查询表中 uid 重复的数据

SELECT id, uid, nameFROM USERWHERE uid IN (SELECT uid FROM USER GROUP BY uid HAVING COUNT(uid) > 1);

2、查询表中重复数据，排除最小id

SELECT id, uid, nameFROM userWHERE uid IN (SELECT uid FROM user GROUP BY uid HAVING COUNT(uid) > 1)AND id NOT IN (SELECT MIN(id) FROM user GROUP BY uid HAVING COUNT(uid) > 1);

3、删除表中重复数据，如果是重复数据，则保留id最小的一条

DELETE FROM USERWHERE id IN (SELECT  u.id FROM (SELECT  id  FROM USER  WHERE uid IN (SELECT   uid  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1)  AND id NOT IN (SELECT   MIN(id)  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1)) AS u);

4、遇到的问题：

一开始直接使用以下语句删除，报错：You can’t specify target table ‘user’ for update in FROM clause

 

DELETE FROM USERWHERE id IN (SELECT  id FROM USER WHERE uid IN (SELECT  uid  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1) AND id NOT IN (SELECT  MIN(id)  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1));

 

查资料后得知：
因为在 MYSQL 里，不能先 select 一个表的记录，在按此条件进行更新和删除同一个表的记录。
解决办法：
将 select 得到的结果，再通过中间表 select 一遍。
SQL如下：

 

DELETE FROM USERWHERE id IN (SELECT  u.id FROM (SELECT  id  FROM USER  WHERE uid IN (SELECT   uid  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1)  AND id NOT IN (SELECT   MIN(id)  FROM USER  GROUP BY uid  HAVING COUNT(uid) > 1)) AS u);