Re: A challenge for the SQL gurus out there...

On Mon, Sep 8, 2008 at 9:49 PM, Merlin Moncure <mmoncure@gmail.com> wrote:
> On Sun, Sep 7, 2008 at 6:09 AM, Gregory Stark <stark@enterprisedb.com> wrote:
>> "Uwe C. Schroeder" <uwe@oss4u.com> 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