Join tables by nearest date?

In response to Nick :

Is it possible to join two tables by the nearest date? For example how
could I find out where the user was on their birthday?

users (id, name, birthday)
1 | one | 2009-07-27
2 | two | 2009-07-28
3 | three | 2009-07-29
4 | four | 2009-07-30
5 | five | 2009-07-31

users_locations (id, user_id, created)
1 | 1 | 2009-05-21
2 | 1 | 2009-06-21
3 | 1 | 2009-07-21
4 | 2 | 2009-05-10
5 | 2 | 2009-06-10
6 | 2 | 2009-07-10

test=*# select distinct on (u.id) u.name, u.birthday, l.created,
u.birthday - l.created as diff from users u right join users_locations l
on (u.id=l.user_id) order by u.id, diff;
name | birthday | created | diff
------+------------+------------+------
one | 2009-07-27 | 2009-07-21 | 6
two | 2009-07-28 | 2009-07-10 | 18
(2 rows)

Helps that?

Andreas
--
Andreas Kretschmer
Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header)
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On Mon, Jul 27, 2009 at 10:51:00PM -0700, Nick wrote:

Is it possible to join two tables by the nearest date? For example how
could I find out where the user was on their birthday?

DISTINCT ON[1]http://www.postgresql.org/docs/current/static/sql-select.html#SQL-DISTINCT is normally the easiest way:

SELECT DISTINCT ON (u.id) u.id, l.id, l.created
FROM users u, users_locations l
WHERE u.id = l.user_id
AND u.birthday <= l.created
ORDER BY u.id, l.created

Untested, but hopefully gives enough hints about where to look!

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

[1]: http://www.postgresql.org/docs/current/static/sql-select.html#SQL-DISTINCT

In response to Sam Mason :

On Mon, Jul 27, 2009 at 10:51:00PM -0700, Nick wrote:

Is it possible to join two tables by the nearest date? For example how
could I find out where the user was on their birthday?

DISTINCT ON[1] is normally the easiest way:

SELECT DISTINCT ON (u.id) u.id, l.id, l.created
FROM users u, users_locations l
WHERE u.id = l.user_id
AND u.birthday <= l.created

AND u.birthday >= l.created

Regards, Andreas
--
Andreas Kretschmer
Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header)
GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net