当转换in 与exists语句时碰上group by

有如下语句，想把他转换成exists语句：外表内表用的都是同一张表

select * from shuxue_new as a where id in  (select max(id) from shuxue_new group by userid);

select * from shuxue_new as a where id not in  (select max(id) from shuxue_new group by userid);

于是：

select * from shuxue_new as a where exists  (select max(id),userid,count(*) from shuxue_new as b where a.id=b.id  group by userid);

select * from shuxue_new as a where not exists  (select max(id),userid,count(*) from shuxue_new as b where a.id=b.id  group by userid);

结果很悲剧，不管怎么调整exists子句，结果依旧不是正确的！原因是因为上面exists子句中有group by！

而在一条SQL语句执行过程中，where条件是在group by与选取select行前面执行的，所以上面的exists子句在还没分组前已经执行了a.id=b.id，因为是同一张表，所以a.id=b.id肯定是返回true的！

另外像select max(id),userid,count(*) from shuxue_new as b where a.id=b.id（ 或者等于一个常量） group by userid) 这种语句本身就有问题！因为where条件里指定了常量等条件，你再group by ，再select max(id) 已经是没有意义的了！最后查询出的永远都只是符合where条件的那几列！

所以像这种使用了group by，或者select max()等函数的in 语句是不能方便的转换成existsy语句的！！

delete a from shuxue_new as a left join (select max(id) as id from shuxue_new  group by userid) as b on a.id=b.id where b.id is null;