---统计各个psam卡号的数量（oracle&mysql）。
SELECT psamcode, count( * ) AS count
FROM cms_psamcard
GROUP BY psamcode
ORDER BY count DESC
LIMIT 20

  ---oracle
delete from stu a where rowid < (select max(b.rowid) from stu b where a.no=b.no and a.name = b.name
1.rowid只是A和B表中的一个字段，估计是整数型字段；
2.这个命令是删除a表中的rowid字段小于b表中与A表中相同no.和name字段最大rowid值的记录

  ---oracle查询表中重复记录的方法（mysql中要使用rowid可创建一个整型唯一自增字段）
1、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)   
2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录

delete from people   
where peopleId in (select peopleId from people group by peopleId   having count(peopleId) > 1)   
and rowid not in (select min(rowid) 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 vitae a   
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)   
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录

delete from vitae a   
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)   
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)   
5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录

select * from vitae a   
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)   
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)