To: pgsql-sql@postgresql.org
From: Harald Fuchs <hari.fuchs@gmail.com>
Subject: Re: self join
Date: Sun, 15 May 2011 10:04:30 +0200
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	<iqne7u$ssa$2@reversiblemaps.ath.cx>
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In article <iqne7u$ssa$2@reversiblemaps.ath.cx>,
Jasen Betts <jasen@xnet.co.nz> writes:

> On 2011-05-14, Seb <spluque@gmail.com> wrote:
>> Hi,
>> 
>> This probably reflects my confusion with how self joins work.
>> 
>> Suppose we have this table:

>> If I want to get a table with records where none of the values in column
>> b are found in column a, I thought this should do it:

> use the "NOT IN" operator with a subquery to retch the disallowed
> values.

> select * from tmp where a NOT IN (select b from tmp);

The third way is an OUTER JOIN:

SELECT t1.a, t1.b
FROM tmp t1
LEFT JOIN tmp t2 ON t2.b = t1.a
WHERE t2.b IS NULL;