使用窗口函数计算百分位数

Question

我知道你可以使用窗口函数获得一部分数据的平均值,总数,最小值和最大值.但是,有可能得到中位数,或第25百分位而不是窗函数的平均值？

换句话说,我如何重写这个以获得每个区域内的ID和第25或第50百分位销售数字而不是平均值？

SELECT id, avg(sales)
    OVER (PARTITION BY district) AS district_average
FROM t

Answer 1

您可以使用percentile_cont()或将其写为聚合函数percentile_disc():

select district, percentile_cont(0.25) within group (order by sales)
from t
group by district;

不幸的是,Postgres目前不支持这些作为窗口函数:

select id, percentile_cont(0.25) within group (order by sales) over (partition by district) 
from t;

所以,你可以使用join:

select t.*, p_25, p_75
from t join
     (select district,
             percentile_cont(0.25) within group (order by sales) as p_25,
             percentile_cont(0.75) within group (order by sales) as p_75
      from t
      group by district
     ) td
     on t.district = td.district