降序排列，limit k 输出top k
获得积分最多的人(一)
https://www.nowcoder.com/practice/1bfe3870034e4efeb4b4aa6711316c3b
/* 子查询
select name,grade_num
from(
select
name,
sum(grade_num) grade_num
from user u
right join grade_info g on u.id=g.user_id
group by name
) t
where grade_num=(
select max(grade_num)
from (
select
name,
sum(grade_num) grade_num
from user u
right join grade_info g on u.id=g.user_id
group by name
) t
)
窗口函数、limit输出第1个 */
select
name,
sum(grade_num) grade_num
from user u
right join grade_info g on u.id=g.user_id
group by name
order by grade_num desc
limit 1
