有一个表user，字段分别有id、nick_name、password、email、phone。

一、单字段（nick_name）

查出所有有重复记录的所有记录

select * from user where nick_name in (select nick_name from user group by nick_name having count(nick_name)>1);

查出有重复记录的各个记录组中id最大的记录

select * from user where id in (select max(id) from user group by nick_name having count(nick_name)>1);

查出多余的记录，不查出id最小的记录

select * from user where nick_name in (select nick_name from user group by nick_name having count(nick_name)>1) and id not in (select min(id) from user group by nick_name having count(nick_name)>1);

删除多余的重复记录，只保留id最小的记录

delete from user where nick_name in (select nick_name from (select nick_name from user group by nick_name having count(nick_name)>1) as tmp1) and id not in (select id from (select min(id) from user group by nick_name having count(nick_name)>1) as tmp2);

二、多字段（nick_name,password）

查出所有有重复记录的记录

select * from user where (nick_name,password) in (select nick_name,password from user group by nick_name,password where having count(nick_name)>1);
 

查出有重复记录的各个记录组中id最大的记录

select * from user where id in (select max(id) from user group by nick_name,password where having count(nick_name)>1);

查出各个重复记录组中多余的记录数据，不查出id最小的一条

select * from user where (nick_name,password) in (select nick_name,password from user group by nick_name,password having count(nick_name)>1) and id not in (select min(id) from user group by nick_name,password having count(nick_name)>1);

删除多余的重复记录，只保留id最小的记录

delete from user where (nick_name,password) in (select nick_name,password from (select nick_name,password from user group by nick_name,password having count(nick_name)>1) as tmp1) and id not in (select id from (select min(id) id from user group by nick_name,password having count(nick_name)>1) as tmp2);

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