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

Quite a clear question :)

SELECT t1.* FROM purchases t1
LEFT JOIN purchases t2
ON t1.address_id = t2.address_id AND t1.purchased_at < t2.purchased_at
WHERE t2.purchased_at IS NULL
ORDER BY t1.purchased_at DESC

And most likely a faster approach:

SELECT t1.* FROM purchases t1
JOIN (
    SELECT address_id, max(purchased_at) max_purchased_at
    FROM purchases
    GROUP BY address_id
) t2
ON t1.address_id = t2.address_id AND t1.purchased_at = t2.max_purchased_at
ORDER BY t1.purchased_at DESC