On Jun 21, 2007, at 11:57 , Josh Tolley wrote:
On 6/21/07, danmcb <danielmcbrearty@xxxxxxxxx> wrote:
Hi
I have two tables, say A and B, that have a many-to-many
relationship, implemented in the usual way with a join table A_B.
How can I economically find all the rows in table A whose id's are
not
in A_B at all (i.e. they have zero instances of B associated)?
Use a left join. For instance, say there are a.id and b.id columns,
which are the primary keys in A and B respectively. Also say A_B
contains columns aid and bid which reference a.id and b.id
respectively.
SELECT * FROM A LEFT JOIN A_B ON (A.ID = A_B.AID) WHERE A_B.BID IS
NULL;
Alternatively you can use EXCEPT. Using Josh's schema:
SELECT id
FROM A
EXCEPT
SELECT aid
FROM A_B.
You'll want to check with EXPLAIN ANALYZE, but in general I suspect
the outer join is faster.
Michael Glaesemann
grzm seespotcode net
