Re: finding items with 0 rels for a 0 to many relationship - Mailing list pgsql-general

On 6/21/07, danmcb <danielmcbrearty@gmail.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;

- Josh