Message-ID: <4C48230D.7030304@archonet.com>
Date: Thu, 22 Jul 2010 11:53:01 +0100
From: Richard Huxton <dev@archonet.com>
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To: "A. Kretschmer" <andreas.kretschmer@schollglas.com>
CC: pgsql-sql@postgresql.org
Subject: Re: grouping subsets
References: <i28ubc$ie9$1@dough.gmane.org>
	<20100722100226.GE10348@a-kretschmer.de>
In-Reply-To: <20100722100226.GE10348@a-kretschmer.de>
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On 22/07/10 11:02, A. Kretschmer wrote:
> In response to Rainer Stengele :
>> What I want to get is the values grouped by "subset", where a subset is a set of rows with identical column until the colum changes.
>> Is there a way to get
>>
>> | 2 | B |
>> | 4 | C |
>> | 4 | B |
>> | 3 | D |
>>
>> by SQL only?
>
> I think, the problem is that there are 2 identical groups. I think, you
> can write a pl/pgsql-proc, selecting all ordered by the date-field and
> walking through the result to do the grouping, checking if the 2nd
> column is different from the previous.
>
> With plain SQL it's maybe possible too, but i don't know how ...

It should be do-able in 8.4 onwards, look into windowing functions. In 
particular the lag() function:

SELECT
     mycode,
     mydate,
     lag(mycode) OVER (ORDER BY mydate) AS prev_code
FROM
     mytable
ORDER BY mydate;

It should be possible to use that as a subquery with an outer query that 
compares mycode=prev_code to get a run length.

-- 
   Richard Huxton
   Archonet Ltd