假设模型表是:

1. 根据单字段过滤:

SELECT user_name, COUNT(*) as count
FROM sys_user
GROUP BY user_name
HAVING count > 1;

结果:

2. 根据多个字段查询重复数据

SELECT user_name, email, COUNT(*) as count
FROM sys_user
GROUP BY user_name, email
HAVING count > 1;

结果:

3. 对一个字段查找重复记录

SELECT * FROM sys_user WHERE user_name IN(
SELECT user_name FROM sys_user GROUP BY user_name HAVING COUNT(user_name) > 1
);

结果:

4. 删除重复记录，只保留user_id字段值最大的记录

delete from sys_user where user_id not in (select maxid from (select max(user_id) as maxid from sys_user group by user_name) b);

结果:

5. 对多个字段查找重复记录

SELECT* 
FROM( SELECT *, CONCAT( user_name, email ) AS nameAndEmail FROM sys_user ) t 
WHEREt.nameAndEmail IN (SELECTnameAndEmail FROM( SELECT CONCAT( user_name, email ) AS nameAndEmail FROM sys_user ) tt GROUP BYnameAndEmail HAVINGcount( nameAndEmail ) > 1 )

结果:

6. 删除重复记录，只保留user_id字段值最大的记录

DELETE 
FROMsys_user 
WHEREuser_id NOT IN (SELECTmaxid FROM( SELECT MAX( user_id ) AS maxid, CONCAT( user_name, email ) AS nameAndEmail FROM sys_user GROUP BY nameAndEmail ) t )

结果:

7. 完全重复数据删除

1.查询表完全重复的纪录,即表行数据是完全重复
select distinct * from sys_user;
2. 如果该表需要删除重复的记录（重复记录保留1条）select distinct * into sys_user1 from sys_user;drop table sys_user;select * into sys_user1 from sys_user1;drop table sys_user1;

8. 关键字段重复删除

1 创建一个临时表，用于存储要删除的重复数据。CREATE TABLE sys_user1 as(SELECT MIN(user_id) as userid from sys_user GROUP BY user_name );SELECT * from sys_user1;DELETE from sys_user where user_id not in(SELECT * from sys_user1);DROP TABLE  sys_user1;