1.查询出所有数据进行分组之后，和重复数据的重复次数的查询数据，先列下：
select count(username) as '重复次数',username from xi group by username having count(*)>1 order by username desc
2.查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
3.查找表中多余的重复记录（多个字段）
select * from people a where (a.peopleId,a.seq) in (select peopleId,seq from people group by peopleId,seq having count(*) > 1)