使用SQL查询数据库中重复的数据的的三种方法

第一种方法

select username count(*) u from user_info group by username having u>1

第二种方法

select username count(*) from user_info group by username having count(username)>1

第三种方法，查询速度较慢

select username from user_info where username = (select username from user_info group by username having count(username)>1