1、单子段（nick_name）
查询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);
查询出有重复记录的各个记录组中的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);
查询出多余的记录，不查出id最小的记录
select * from user where id in (select max(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);

2、对字段
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);
查处各个重复记录组中多余的鸡柳数据，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);
删除多有的重复记录，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);