1、从一张表中查询出，相同id的数据的所有信息
select * from 表 where id in（select id from 表 group by id having count（*）>1）；
2、从一张表中删除，code重复的数据
delete from area a where a.id>(select min(id) from area b where a.code=b.code )；
3、从一张表中查询出，去掉code重复的数据
select * from area a where a.id in (select min(id) from area b where a.code=b.code);