Select entire row on group by aggregation

Question

I'm having some struggle with something that should be a simple SQL query.

This is my initial database schema:

Also prepared the example in SQLFiddle

The query I've ended up with is:

select
       b.ad_id,
       b.auction_id,
       max(b.amount) as max,
       max(b.created_at) created_at
from bid b
where b.user_id = '601'
group by b.ad_id, b.auction_id

But in the result, I need the whole row from the bid table:

select
       b.id,
       b.ad_id,
       b.auction_id,
       max(b.amount) as max,
       max(b.created_at) created_at
from bid b
where b.user_id = '601'
group by b.ad_id, b.auction_id

Which fails with: [42803] ERROR: column "b.id" must appear in the GROUP BY clause or be used in an aggregate function Position: 16 . Cannot add the id field in the GROUP BY clause, because it will add some extra rows I don't need.

What I need is to select from the bid table the highest record (amount field) grouped by auction_id and ad_id.

I think I need to make some self inner join or subselect but right now I'm not able to write the SQL.

Answer 1

What I need is to select from the bid table the highest record (amount field) grouped by auction_id and ad_id

Take a look at DISTINCT ON in the docs. Your desired result would be obtained by the following query.

select DISTINCT ON (b.ad_id, b.auction_id)
       b.id,
       b.ad_id,
       b.auction_id,
       b.amount
       b.created_at
from bid b
where b.user_id = '601'
ORDER BY b.ad_id, b.auction_id, b.amount DESC

Answer 2

If you want the most recent row for each auction for the given user, then you can use a correlated subquery to filter:

select b.*
from bid b
where b.user_id = '601' and
      b.created_at = (select max(b2.created_at)
                      from bid b2
                      where b2.auction_id = b.auction_id and
                            b2.user_id = b.user_id
                     );

This seems like a sensible interpretation of what you want. I don't know if ad_id is needed.