【MySQL写法细节】那些不知不觉就会犯的错误

SELECT u.name,g.grade_num
from (select user_id,sum(grade_num) grade_num
      from grade_info
      group by user_id
      ) g
join `user` u on g.user_id = u.id
where g.grader_num = (SELECT max(g.grade_num) from g)

这种写法有个错误：
为什么不能在子查询的表格基础上，再做一次子查询？？？