标签：rac ora date net AC font not 文章 sel
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
标签：rac ora date net AC font not 文章 sel
原文地址：https://www.cnblogs.com/shihuibei/p/8809908.html
