Selecting rows ordered by some column and distinct on another

Question

Related to - PostgreSQL DISTINCT ON with different ORDER BY

I have table purchases (product_id, purchased_at, address_id)

Sample data:

| id |         


        

Answer 1

Your ORDER BY is used by DISTINCT ON for picking which row for each distinct address_id to produce. If you then want to order the resulting records, make the DISTINCT ON a subselect and order its results:

SELECT * FROM
(
  SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
  FROM "purchases"
  WHERE "purchases"."product_id" = 2
  ORDER BY purchases.address_id ASC, purchases.purchased_at DESC
) distinct_addrs
order by distinct_addrs.purchased_at DESC