Percentile calculation with a window function

Question

I know you can get the average, total, min, and max over a subset of the data using a window function. But is it possible to get, say, the median, or the 25th percentile instead of the average with the window function?

Put another way, how do I rewrite this to get the id and the 25th or 50th percentile sales numbers within each district rather than the average?

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

Answer 1

You can write this as an aggregation function using percentile_cont() or percentile_disc():

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

Unfortunately, Postgres doesn't currently support these as a window functions:

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

So, you can use a 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