https://blog.csdn.net/eagle89/article/details/90901755
假设有一个表user，字段分别有id–nick_name–password–email–phone，分情况如下（注意删除多余记录时要创建临时表，不然会报错）：
一、单字段（nick_name）
1、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);
2、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);
3、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);
4、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）
1、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);
2、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);
3、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);
4、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);