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Subject: Re: SQL challenge--top 10 for each key value?
From: Rod Taylor <pg@rbt.ca>
To: Greg Stark <gsstark@mit.edu>
Cc: Jeff Boes <jboes@nexcerpt.com>, pgsql-sql@postgresql.org
In-Reply-To: <87r7uyf3ki.fsf@stark.xeocode.com>
References: <33af4fa4e4ec230574cefcff737ea40a@news.teranews.com>
	<87r7uyf3ki.fsf@stark.xeocode.com>
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Date: Thu, 08 Apr 2004 23:31:07 -0400
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On Thu, 2004-04-08 at 19:33, Greg Stark wrote:
> Jeff Boes <jboes@nexcerpt.com> writes:
> 
> > I headed off in the direction of groups of SELECTs and UNIONs, and quit when I
> > got to something like four levels of "SELECT ... AS FOO" ...
> 
> four? wimp, that's nothing!
> 
> ok, seriously I think there's no way to do this directly with straight SQL.
> You would have to define a non-immutable function that has some temporary
> storage where it keeps track of how many it has seen. 

I don't believe that is true, though it is certainly is in PostgreSQL.

The spec has the ability to apply a progressive aggregate on the results
of a query (window function). Meaning you can accomplish things like
counting (ROW_NUMBER) or running totals.

Something along the lines of the below would accomplish what you want
according to spec. ROW_NUMBER() is a spec defined function. (6.10 of
SQL200N)

        SELECT * 
          FROM (SELECT ROW_NUMBER() OVER (DISTINCT query) AS counter
                  <rest of query>
               )
          WHERE counter > 10;