From: Markus Bertheau <twanger@bluetwanger.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-sql <pgsql-sql@postgresql.org>
Subject: Re: Difference between IN and JOIN
Date: Mon, 04 Oct 2004 16:23:49 +0200
Message-ID: <1096899829.2651.13.camel@dicaprio.akademie1.de> (raw)
In-Reply-To: <20688.1096899436@sss.pgh.pa.us>
References: <1096879190.2651.8.camel@dicaprio.akademie1.de>
<20688.1096899436@sss.pgh.pa.us>
В Пнд, 04.10.2004, в 16:17, Tom Lane пишет:
> Markus Bertheau <twanger@bluetwanger.de> writes:
> > I lately wondered if there is a difference between a JOIN and a IN in
> > queries similar to the following:
>
> > SELECT f1 FROM t1 JOIN t2 ON (t.f2 = t2.f2) WHERE t2.f3 = x
>
> > SELECT f1 FROM t1 WHERE t1.f2 IN (SELECT f2 FROM t2 WHERE f3 = x)
>
> > As I see it there's no semantic difference between the two.
>
> There's plenty of difference, if t2 contains repeated occurrences of f2
> values.
In my case f2 is the primary key of t2. I probably didn't state the
point of my question well enough: You can write an IN query as a
semantically equal JOIN. I wondered, which one I should prefer, and on
which grounds.
Thanks
--
Markus Bertheau <twanger@bluetwanger.de>
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To: pgsql-sql@postgresql.org
Cc: twanger@bluetwanger.de, tgl@sss.pgh.pa.us
Subject: Re: Difference between IN and JOIN
In-Reply-To: <1096899829.2651.13.camel@dicaprio.akademie1.de>
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