X-Original-To: pgsql-sql-postgresql.org@localhost.postgresql.org
Received: from localhost (unknown [200.46.204.144])
	by svr1.postgresql.org (Postfix) with ESMTP id 161F752939
	for <pgsql-sql-postgresql.org@localhost.postgresql.org>;
	Wed,  6 Jul 2005 07:30:29 -0300 (ADT)
Received: from svr1.postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 54834-04
	for <pgsql-sql-postgresql.org@localhost.postgresql.org>;
	Wed,  6 Jul 2005 10:30:27 +0000 (GMT)
Received: from mail.metronet.co.uk (mail.metronet.co.uk [213.162.97.75])
	by svr1.postgresql.org (Postfix) with ESMTP id 378CC528BE
	for <pgsql-sql@postgresql.org>; Wed,  6 Jul 2005 07:30:26 -0300 (ADT)
Received: from mainbox.archonet.com
	(84-51-143-99.archon037.adsl.metronet.co.uk [84.51.143.99])
	by smtp.metronet.co.uk (MetroNet Mail) with ESMTP
	id 3D15840D4D3; Wed,  6 Jul 2005 11:30:22 +0100 (BST)
Received: from localhost (localhost.localdomain [127.0.0.1])
	by mainbox.archonet.com (Postfix) with ESMTP id DE3C1FFA7;
	Wed,  6 Jul 2005 11:30:16 +0100 (BST)
Received: from mainbox.archonet.com ([127.0.0.1])
	by localhost (mainbox [127.0.0.1]) (amavisd-new, port 10024)
	with ESMTP id 15469-10; Wed, 6 Jul 2005 11:30:12 +0100 (BST)
Received: from [192.168.1.17] (client17.office.archonet.com [192.168.1.17])
	by mainbox.archonet.com (Postfix) with ESMTP id B320BFF8B;
	Wed,  6 Jul 2005 11:30:07 +0100 (BST)
Message-ID: <42CBB2AF.2010601@archonet.com>
Date: Wed, 06 Jul 2005 11:30:07 +0100
From: Richard Huxton <dev@archonet.com>
User-Agent: Mozilla Thunderbird 1.0.2 (X11/20050317)
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Grant Morgan <grant@ryuuguu.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: left joins
References: <op.sthktgypuogdv0@localhost.localdomain>
In-Reply-To: <op.sthktgypuogdv0@localhost.localdomain>
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: 7bit
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, hits=0.056 tagged_above=0 required=5 tests=AWL,
	FORGED_RCVD_HELO
X-Spam-Level: 
X-Archive-Number: 200507/31
X-Sequence-Number: 22009

Grant Morgan wrote:
> I am having a problem with left joins in Postgresql.(probably my 
> misunderstanding of left joins)
> 
> My first Query returns
> 70,000
> 
> select count(*)
> from  h
> where h.tn > 20
> and h.tn < 30
> 
> my left join
> returns only 34,000
> 
> select count(*)
> from  h left join p using (r,pos)
> where h.tn > 20
> and h.tn < 30
> and p.r_order=1
> 
> since it is a left join I though I should get a number no smaller in the 
> left join than the original unjoined query.

You don't. You make the left join, then apply another condition 
"p.r_order=1". If you want to demonstrate a problem, you'll need to make 
sure the queries are the same.

-- 
   Richard Huxton
   Archonet Ltd