1、查询表中重复数据，根据一个字段来判断提取

select * from sly_news 
where username in (select username from sly_news group by username having count(username) > 1)

2、查询表中的重复数据，不提取最小id值的数据， 主要用来删除重复值，删除替换最前面的select * 为delete 即可

select * from sly_news 
where username in (select username from sly_news group by username having count(username) > 1 
and id not in (select min(id) from sly_news group by username having count(username) > 1))

3、查询表中重复数据， 根据多个字段判断

select * from sly_news a where (a.username, a.userid) in (
select username, userid from sly_news group by username, userid having count(*) > 1 )