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From: Richard Huxton <dev@archonet.com>
To: Grant Morgan <grant@ryuuguu.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: left joins
Date: Wed, 06 Jul 2005 11:30:07 +0100
Message-ID: <42CBB2AF.2010601@archonet.com> (raw)
In-Reply-To: <op.sthktgypuogdv0@localhost.localdomain>
References: <op.sthktgypuogdv0@localhost.localdomain>

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



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