有一個表user，字段分別有id、nick_name、password、email、phone。
一、單字段（nick_name）
查出所有有重復記錄的所有記錄
select * from user where nick_name in (select nick_name from user group by nick_name having count(nick_name)>1);
查出有重復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);
查出多余的記錄，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);
刪除多余的重復記錄，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);
二、多字段（nick_name,password）
查出所有有重復記錄的記錄
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);
查出有重復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);
查出各個重復記錄組中多余的記錄數據，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);
刪除多余的重復記錄，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);
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