mysql 删除重复记录

这不是之前做了一个磁力小站么.. 发现之前好多记录都是重复的（不要问我怎么发现的），就删除下，感觉不难，但总是写不对.. 这里纯粹记下sql.

查询重复记录

select infohash from demo_magnet group by infohash having count(1) > 1;

删除重复记录（所有）

//请注意：这里是全部删除，不会保留哈，一定要看仔细了... 
delete from demo_magnet where infohash in (
	select infohash from (
		select infohash from demo_magnet group by infohash having count(1) > 1
	) t
) ;
//注意哈,要带t(作为一个表)

保留一条记录

delete from demo_magnet where infohash in (
	select infohash from (
		select infohash from demo_magnet group by infohash having count(1) > 1
	) t
) 
and id not in (
	select dt.minid from (
		select min(id) as minid from demo_magnet group by infohash having count(1) > 1
	) dt
)

这里我的ID 是自增的可以使用这个。如果没有ID可以来判定怎么办？

用 _rowid 和 oracle 的 rowid 类似

delete from demo_magnet where infohash in (
	select infohash from (
		select infohash from demo_magnet group by infohash having count(1) > 1
	) t
) 
and _rowid not in (
	select dt.minid from (
		select min(_rowid) as minid from demo_magnet group by infohash having count(1) > 1
	) dt
)

以上。