MySQL删除重复数据只保留一条

Posted 2020-10-30 vczh

 面试碰到一个mysql的有趣的题目，如何从student表中删除重复名字的行，并保留最小id的记录？

很遗憾当时没有做出来，回家搜索了一番，发现利用子查询的可以很快解决。

1、删除表中多余的重复记录，重复记录是username判断，只留有id最小的记录

delete from studentwhere
username in ( select username from studentgroup by username having count(username)>1)
and id not in (select min(id) as id from studentgroup by username having count(username)>1 )

(上面这条语句在mysql中执行会报错：

执行报错：1093 - You can‘t specify target table ‘student‘ for update in FROM clause

原因是：更新数据时使用了查询，而查询的数据又做了更新的条件，mysql不支持这种方式。oracel和msserver都支持这种方式。

怎么规避这个问题？

再加一层封装,

delete from student where
username in (select username from ( select username from student group by username having count(username)>1) a)
and id not in ( select id from (select min(id) as id from student group by username having count(username)>1 ) b)

 注意select min(id) 后面要有as id.

其实还有更简单的办法（针对单个字段）：

delete from student where
id not in (select id from (select min(id) as id from student group by username) b);

 

拓展：

2、删除表中多余的重复记录（多个字段），只留有id最小的记录

delete from student a
where (a.username,a.seq) in (select username,seq from (select username,seq from a group by username,seq having count(*) > 1)  t1)
and id not in ( select id from (select min(id) from vitae group by username,seq having count(*)>1) t2)

参考文章：

https://blog.csdn.net/anya/article/details/6407280