当某数的正逆序累计均大于等于整个序列数字个数的一半即为中位数

select grade
from (
    select grade,
    (select sum(number) from class_grade) as total,
    sum(number)over(order by grade) a,
    sum(number)over(order by grade desc) b
    from class_grade
) t
where a >= total/2 and b >= total/2
order by grade