sql查询重复记录并统计重复次数

（1）sql查询重复记录并统计重复次数

select userName,count(*)重复次数 from user group by userName having count(*)>1

（2）查询重复记录明细

select * from user

where userName in (select userName from  user group  by  userName having  count(userName) > 1)

(3) 只保留一条数据

--删除重复数据，即从重复的数据中，删除掉需要保留数据之外的所有信息：

DELETE 
FROM
    dept 
WHERE
    dname IN ( SELECT * FROM ( SELECT dname FROM dept GROUP BY dname HAVING count( dname ) > 1 ) a ) 
    AND deptno NOT IN ( SELECT * FROM ( SELECT min( deptno ) AS deptno FROM dept GROUP BY dname HAVING count( dname ) > 1 ) b )