降序排列，limit k 输出top k

/* 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