Get most recent message between users?

On Mon, Aug 17, 2009 at 04:33:11PM -0700, Nick wrote:

messages (id, to_user, from_user, created)
1 | 1 | 2 | 2009-05-21
2 | 1 | 3 | 2009-06-21
3 | 1 | 4 | 2009-07-21
4 | 2 | 1 | 2009-05-10
5 | 2 | 3 | 2009-06-10
6 | 1 | 2 | 2009-07-10

How could I get the most recent message between two users?

More details would help, but failing that how about:

SELECT DISTINCT ON (to_user, from_user) id, to_user, from_user, created
FROM messages
ORDER BY to_user, from_user, created DESC;

That would only chop out id=1 from the above list.

--
Sam http://samason.me.uk/

Nick wrote:

messages (id, to_user, from_user, created)
1 | 1 | 2 | 2009-05-21
2 | 1 | 3 | 2009-06-21
3 | 1 | 4 | 2009-07-21
4 | 2 | 1 | 2009-05-10
5 | 2 | 3 | 2009-06-10
6 | 1 | 2 | 2009-07-10

How could I get the most recent message between two users?

Depends on what you want (all untested).

If you want the most recent message from user 1 to user 2 then:
SELECT ... where from_user=1 and to_user=2 order by created desc limit 1;

If you want the most recent message _between_ two users (either
direction), then you need to include both directions in the where clause:
where (from_user=1 and to_user=2) or (from_user=2 and to_user=1)...

If you want the most recent message time for all messages for all
user-pairs (one direction)
...to_user, from_user, max(created)...group by to_user,from_user

Or most-recent between all user-pairs:
...int4smaller(from_user,to_user), int4larger(from_user,to_user),
max(created) group by int4smaller(from_user,to_user),
int4larger(from_user,to_user)

Optimizing some of these could be "fun", though.

Cheers,
Steve

Thank you and sorry for the vagueness. The answer I was looking for
was Steves last suggestion - getting the newest messages between all
users which would return...

6 | 1 | 2 | 2009-07-10
5 | 2 | 3 | 2009-06-10
3 | 1 | 4 | 2009-07-21
2 | 1 | 3 | 2009-06-21

On Aug 17, 6:03 pm, scrawf...@pinpointresearch.com (Steve Crawford)
wrote: