多子查询而已，没啥难度，就是基本的查询

select
k.user_id,u.name,k.grade_sum
from
(select
t.user_id,t.grade_sum,rank()over(order by t.grade_sum desc) ranking
from
(select
user_id,SUM(grade_num) grade_sum
from
grade_info
where
type="add"
group by
user_id)t)k,
user u
where
k.user_id=u.id
and
k.ranking=1