SQL 查询问题 group by

Question

student(sno,sname,,sdept)
course(cno,cname,ccredit)
sc(sno,cno,grade)
为什么这样写不行呢？
//查询CS系成绩最高的学生的学号、姓名以及成绩

select student.sno,sname,grade from student,sc where student.sno = sc .sno
and grade=(select max(grade) from sc where sc.sno = sno and sdept = 'cs')
还有一个问题;
//查询每个系的最高成绩
select sdept,max(grade) from sc,student where sc.sno=student.sno group by sdept;这样写把每个系的最高分列出来了，当我想把sno列出来，所以加上了sc.sno，下面的就什么结果也没有，是错的。
--select sdept,sc.sno,max(grade) from sc,student where sc.sno=student.sno group by sdept,sc.sno
那这样的问题的group by到底该怎么写呢？如果我想把学号等也列出来是不是只可以写成相关查询呢？
select x.sno,sdept,grade from student x,sc where x.sno = sc.sno and grade =
(select max(grade) from sc,student where sc.sno=student.sno and sdept=x.sdept) //这个结果又是对的
谢谢指导。

Answer 1

 select student.sno,sname,grade from student,sc where student.sno = sc .sno
and grade=(select max(grade) from sc where sc.sno = sno and sdept = 'cs')
这句不对的原因在于，下面子句中，左边的sdept不存在，在sc表中没有
(select max(grade) from sc where sc.sno = sno and sdept = 'cs')