中位数——首先要得到组内的排序和总的记录数量，然后根据中位数的特点构造筛选条件：t/2 <= rk and rk <= t/2 + 1， 例：奇数记录排序后1,2,3 中位数是2，其满足2>= 3/2（1.5）, 2<= 3/2+1（2.5）；偶数记录排序后1,2 中位数是1和2，满足1/2 = 0.5 ，1/2 + 1 = 1.5 和 2/2 =1， 2/2 + 1 = 2，即 0.5 <= 1 <= 1.5,
1 <= 2 <= 2。t/2 <= rk and rk <= t/2 + 1均可适用奇偶情况。

select a.id, a.job, a.score, a.rk from
(select *, row_number() over(partition by job order by score desc) as rk, 
count(*) over(partition by job) as t
from grade) as a
where t/2 <= rk and rk <= t/2 + 1
order by id asc;