Date: Sun, 18 Jan 2004 21:57:14 +0000
From: Richard Poole <richard@ruthie.org>
To: pgsql-sql@postgresql.org
Subject: Re: Left joins with multiple tables
Message-ID: <20040118215714.GA11708@guests.deus.net>
References: <pan.2004.01.17.02.29.56.311442@cfconsulting.ca>
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On Sat, Jan 17, 2004 at 02:30:01AM +0000, Colin Fox wrote:

> For each person in the people table, they may or may not have a record in
> a, may or may not have a record in b, and may or may not have a record in
> c.

...

> But I'd like to be able to do something like:
> 
> select
>     id, name, a.field1, b.field2, c.field3
> from
>     people p left outer join a on a.person_id = p id,
>     people p left outer join b on b.person_id = p.id,
>     people p left outer join c on c.person_id = p.id;


You can just chain the joins and the Right Thing will happen:

SELECT id, name, a.field1, b.field2, c.field3
FROM
  people p
  LEFT OUTER JOIN a ON (p.id = a.person_id)
  LEFT OUTER JOIN a ON (p.id = b.person_id)
  LEFT OUTER JOIN a ON (p.id = c.person_id)

I'm not sure that this behaviour is mandated by the SQL standard;
a certain other popular open source database-like product interprets
the same construction differently. But it does do what you want in
postgres.

Richard