Re: finding items with 0 rels for a 0 to many relationship

[Michael Glaesemann <grzm(at)seespotcode(dot)net>]

On Jun 21, 2007, at 11:57 , Josh Tolley wrote:

> On 6/21/07, danmcb <danielmcbrearty(at)gmail(dot)com> 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