SQL中位数问题

571. 建立两个累计频率， 升序降序， 中位数等于 平均值 of（当升序累计频率>=总频率/2 且降序累计频率>=总频率/2 时的值）

select
avg(t.number) as median
from
(
select
n1.number,
n1.frequency,
#求累计频率， 建立自连接表
(select sum(frequency) from numbers n2 where n2.number<=n1.number) as asc_frequency,
(select sum(frequency) from numbers n3 where n3.number>=n1.number) as desc_frequency
from numbers n1
) t
where t.asc_frequency>= (select sum(frequency) from numbers)/2
and t.desc_frequency>= (select sum(frequency) from numbers)/2