Message-ID: <3DC194BC.966E95CD@nsd.ca>
Date: Thu, 31 Oct 2002 15:38:20 -0500
From: Jean-Luc Lachance <jllachan@nsd.ca>
MIME-Version: 1.0
To: Richard Huxton <dev@archonet.com>
Cc: Wei Weng <wweng@kencast.com>, pgsql-sql@postgresql.org
Subject: Re: How do you write this query?
References: <1036088486.13986.19.camel@Monet>
	<200210311909.11341.dev@archonet.com>
Content-Type: text/plain; charset=us-ascii
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Thank goodness for nested select!

select data1 from test where data2 = ( select distinct data2 from test
where data1 = 'pooh') and data = 3;

JLL

Richard Huxton wrote:
> 
> On Thursday 31 Oct 2002 6:21 pm, Wei Weng wrote:
> >  data | data1 | data2
> > ------+-------+-------
> >     1 | foo   | bar
> >     2 | greg  | bar
> >     3 | pooh  | bar
> >     4 | dah   | peng
> >
> > I need a query that returns me the "data1" that satisfies the logic of
> > the following pseudo code:
> >
> > 1: select data2 into @out from test where data1 = 'pooh'
> > 2: select data1 from test where data2 = @out and data = 3
> 
> The most literal would be something like:
> 
> SELECT t1.data1 FROM test t1
> WHERE t1.data=3 AND t1.data2 IN
>     (SELECT t2.data2
>     FROM test t2
>     WHERE t2.data1='pooh')
> 
> You can probably get away without the t1/t2 stuff but that should make things
> clear.
> 
> Since Postgresql isn't very good at optimising IN, you might want to rewrite
> it as an EXISTS query instead - see the manuals and mailing list archives for
> details.
> 
> HTH
> --
>   Richard Huxton
> 
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