How to use a SQL window function to calculate a percentage of an aggregate

Question

I need to calculate percentages of various dimensions in a table. I\'d like to simplify things by using window functions to calculate the denominator, however I am having an iss

Answer 1

Do you need to do it all with window functions? Sounds like you just need to group the result you have by d1 and d2 and then sum the sums:

select d1, d2, sum(p)
from (
    select d1, d2, v/sum(v) over (partition by d1) as p
    from test
) as dt
group by d1, d2

That gives me this:

 d1 | d2 |          sum           
----+----+------------------------
 a  | x  | 0.25000000000000000000
 a  | y  | 0.75000000000000000000
 b  | x  | 1.00000000000000000000

Answer 2

I think you are looking for this:

SELECT d1, d2, sum(v)/sum(sum(v)) OVER (PARTITION BY d1) AS share
FROM   test
GROUP  BY d1, d2;

Produces the requested result.

Window functions are applied after aggregate functions. The outer sum() in sum(sum(v)) OVER ... is a window function (attached OVER ... clause) while the inner sum() is an aggregate function.

Effectively the same as:

WITH x AS (
   SELECT d1, d2, sum(v) AS sv
   FROM   test
   GROUP  BY d1, d2
   )
SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM   x;

Or (without CTE):

SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM  (
   SELECT d1, d2, sum(v) AS sv
   FROM   test
   GROUP  BY d1, d2
   ) x;

Or @Mu's variant.

Aside: Greenplum introduced correlated subqueries with version 4.2. See release notes.