MySql数据查重、去重的实现

青旅半醒 2022-02-25 08:46 274阅读 0赞

刚电话面试被问的，但是没有答出来，在此记录一下。

原文：[https://blog.csdn.net/lzc4869/article/details/79128865][https_blog.csdn.net_lzc4869_article_details_79128865]  
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);

[https_blog.csdn.net_lzc4869_article_details_79128865]: https://blog.csdn.net/lzc4869/article/details/79128865