MySql數據查重、去重的實現

一、背景

    假設有一個表user，字段分別有id、nick_name、password、email、phone，分情況如下（注意刪除多餘記錄時要創建臨時表，不然會報錯）。

二、單字段（nick_name）

1、查出所有有重複記錄的所有記錄

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

2、查出有重複記錄的各個記錄組中id最大的記錄

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

3、查出多餘的記錄，不查出id最小的記錄

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、刪除多餘的重複記錄，只保留id最小的記錄

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);

三、多字段（nick_name,password）

1、查出所有有重複記錄的記錄

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、查出有重複記錄的各個記錄組中id最大的記錄

select * from user where id in

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

3、查出各個重複記錄組中多餘的記錄數據，不查出id最小的一條

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、刪除多餘的重複記錄，只保留id最小的記錄

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);