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）
最差是第几名(二)
https://www.nowcoder.com/practice/165d88474d434597bcd2af8bf72b24f1
/*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
