中位数：正序、倒序，取值均大于总数的1/2，即为中位数（子查询）

/*创建辅助列：正序、倒序，取值均大于总数的1/2，即为中位数   */
select grade
from (select 
        grade,
        sum(number) over(order by grade) a,
        sum(number) over(order by grade desc) b,
        (select sum(number) from class_grade) total
    from class_grade) t
where a>=total/2 and b>=total/2
order by grade