Re: Join issue on a maximum value

On Wed, Apr 21, 2004 at 02:29:34PM -0400, Heflin wrote:
> OK, it's been a while since I've had to do anything remotely complex in
> SQL, so this may just be a pure brain block on my part.
>
...
>
> So a basic JOIN gets this:
>
> SELECT auction.auction_id, image.image_id, image.image_descr
> FROM auction JOIN image ON auction.auction_id = image.auction_id
> WHERE auction.auction_owner = 'Mabel';
>
> auction_id | image_id | image_descr
> ------------+----------+-------------
> 1 | 1 | image 1
> 1 | 2 | image 2
> 2 | 3 | image 3
> 3 | 4 | image 4
> 3 | 5 | image 5
> 3 | 7 | image 8
> (6 rows)
>
> Now the problem: I can't seem to remember how to get only the max value
> for the image_id for each auction_id so that the result set would be:
>
> auction_id | image_id | image_descr
> ------------+----------+-------------
> 1 | 2 | image 2
> 2 | 3 | image 3
> 3 | 7 | image 8
>
> Playing with the max() aggregate seems to be the correct path, but for
> the life of me I can't seem to get the syntax to the point that it
> produces what I need. Any help would be greatly appreciated!

A simple way to write this is to use DISTINCT ON combined with ORDER
BY. In this case, these clauses can substitute for the MAX aggregate:

select distinct on (auction.auction_id) auction.auction_id, image.image_id,
image.image_descr
from auction join image using (auction_id)
where auction.auction_owner = 'Mabel'
order by auction.auction_id, image.image_id desc;

(I haven't tested this.)

I think you need a subselect in there if you want to use the MAX
aggregate.

- Jeremy