对分组取最大值语句的疑惑??
Select * from T4 a where not exists
(Select * From T4 where email=a.email and score>a.score)
这个语句是求以email分组后中的score最大值~`看不太明白,请达人讲解一下,谢谢!
和下面这句为什么会不同呢?
Select * from T4 where not exists
(Select 1 From T4 a,T4 b where a.email=b.email and b.score>a.score)