sql语句,做了好久了,就是绕不出来

一张表,有班级号,学号和成绩三个字段,怎么用sql查询出每个班级大于平均分的人数,挺急的,明天要用,有人能帮帮忙嘛,谢谢了谢谢了

select score > a._score from grade, (select avg(score) as _score from grade) a

select * from grade, (select avg(score) as _score from grade) a where score > a._score