On Mon, Sep 8, 2008 at 9:49 PM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Sun, Sep 7, 2008 at 6:09 AM, Gregory Stark <stark@xxxxxxxxxxxxxxxx> wrote: >> "Uwe C. Schroeder" <uwe@xxxxxxxxx> writes: >> >>> I want to get a list looking like >>> >>> forum id thread_id post_id >>> 1 6 443 >>> 2 9 123 >>> 3 3 557 >> ... >>> It all boils down to me not being able to come up with a query that gives me >>> the latest post per forum_id. >> >> In a situation like this I would probably denormalize the tables slightly by >> adding a form_id key to the individual posts. That would make it hard to ever >> move a thread from one forum to another, though not impossible, but would help >> in this case as well as any other time you want to do an operation on all >> posts in a forum regardless of thread. > > <sql guru hat on> > > select f.*, > ( > select (t, > ( > select p from post p > where p.thread_id = t.thread_id > order by post_id desc limit 1 > )) > from thread t > where forum_id = f.forum_id > order by thread_id desc limit 1 > ) as threadpost > from forum f; > > :-) > > 'thread post' is a nested composite, ((thread), post). > > The above will pretty much guarantee a fast query unless the number of > forums is large. To pull out the composite fields, wrap in a subquery > or (better yet) fire up libpqtypes. oh, and I assumed id order corresponds to date order...should have mentioned that, and I had forum.id as forum_id, which it should have been in the first place. merlin
