思路:窗口函数
关键：窗口函数在group by后运行，所以可以对聚合后的和进行排序
select
id
,name
grade_sum
from
(
select
id
,name
,sum(grade_num) grade_sum
,dense_rank()over(order by sum(grade_num) desc) rk
from user u
left join grade_info g on u.id = g.user_id
group by 1,2
) a
where rk =1
order by id