假設有一個表user，欄位分別有id–nick_name–password–email–phone，分情況如下（注意刪除多餘記錄時要建立臨時表，不然會報錯）：

一、單欄位（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); 1 2 3 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); 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); 1 2 3 4 5 6 7 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); 1 2 3 4 5 6 7 8 9 10 11 二、多欄位（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); 1 2 3 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); 1 2 3 3、查出各個重複記錄組中多餘的記錄資料，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); 1 2 3 4 5 6 7 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);