Date: Thu, 22 Jul 2010 12:02:26 +0200
From: "A. Kretschmer" <andreas.kretschmer@schollglas.com>
To: pgsql-sql@postgresql.org
Subject: Re: grouping subsets
Message-ID: <20100722100226.GE10348@a-kretschmer.de>
References: <i28ubc$ie9$1@dough.gmane.org>
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In response to Rainer Stengele :
> Hi,
> 
> having a table similar to
> 
> | 1 | B | [2010-07-15 Do] |
> | 1 | B | [2010-07-16 Fr] |
> |---+---+-----------------|
> | 2 | C | [2010-07-17 Sa] |
> | 2 | C | [2010-07-18 So] |
> |---+---+-----------------|
> | 1 | B | [2010-07-19 Mo] |
> | 1 | B | [2010-07-20 Di] |
> | 1 | B | [2010-07-21 Mi] |
> | 1 | B | [2010-07-22 Do] |
> |---+---+-----------------|
> | 3 | D | [2010-07-23 Fr] |
> 
> a simple group by gives me:
> 
> | 6 | B |
> | 4 | C |
> | 3 | D |
> 
> 
> 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 ...

Regards, Andreas
-- 
Andreas Kretschmer
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