找出postgres中列的中位数

Question

我正在计算跨栏的13周值的总和，如下所示，我希望计算中位数的方法可以帮助我吗？谢谢！

select a.prod_name,a.week_date,a."13week_sum",
COALESCE(CASE WHEN COUNT(*) over (partition by prod_name order by prod_name,week_date desc rows between 0 preceding and 12 following) = 13 then 
sum(a."value") over (partition by prod_name order by prod_name,week_date desc rows between 0 preceding and 12 following)
end)
as "13week_sum"
from vin_table_temp a order by week_date

我正在使用8.2版postgres的青梅。