查询以及删除一个数据库表内的重复数据。
1.查询表中的多余的重复记录，重复记录是根据单个字段来判断的。
select * from biao where id in (select id from biao group by id having count(id) >1 )
2.删除表中的多余的重复记录，重复记录是根据（id）来判断，只留rowid 最小值。
delete from biao where id in ( select id from biao group by id having count(id) ) and rowid not in ( select min(rowid) from biao group by id having count(id)>1 )
delete from xuesheng
where T_NAME in
(select T_NAME
from xuesheng
group by T_NAME, T_ADDRESS, T_PHONE, T_BIRTHDAY
having count(*) > 1) --重复数据
and rowid not in
(select max（rowid）
from xuesheng
group by T_NAME, T_ADDRESS, T_PHONE, T_BIRTHDAY
having count(*) > 1) --保留一条重复数据
3.删除表中重复的记录（多个字段），
delete from xuesheng
where T_NAME in
(select T_NAME
from xuesheng
group by T_NAME, T_ADDRESS, T_PHONE, T_BIRTHDAY
having count(*) > 1) --不保留重复数据
来自：https://www.cnblogs.com/252e/archive/2012/09/13/2682817.html