From: Jeff Boes <jboes@qtm.net>
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Subject: Re: Complicated "group by" question
References: <Pine.LNX.4.53.0408251300580.20106@perrin.socsci.unc.edu>
In-Reply-To: <Pine.LNX.4.53.0408251300580.20106@perrin.socsci.unc.edu>
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Message-ID: <IgGZc.92877$JG7.85347@hydra.nntpserver.com>
Date: Thu, 02 Sep 2004 10:39:36 -0400
To: pgsql-sql@postgresql.org

Andrew Perrin wrote:
> I have a table of people ("reviewers"), a table of review assignments
> ("assign"), and a table of review acceptances ("accept"). I would like to
> be able to write a query to return the latest (e.g., max(assign_date))
> assignment for each reviewer, plus the acc_id field from "accept".  I
> think I should be able to do this with a GROUP BY clause, but am having no
> luck.
> 
> Table structure:
> 
> reviewers	assign		accept
> -----------------------------------------
> reviewer_id	assign_id	accept_id
> 		reviewer_id	assign_id
> ...		assign_date
> 		...		...
> 


I think you want to write a non-GROUPed query using "DISTINCT ON". 
Something like this:

SELECT DISTINCT ON (reviewer_id,assign_id)
  reviewer_id,
  assign_id,
  assign_date,
  accept_id
FROM reviewers
JOIN assign USING (reviewer_id)
JOIN accept USING (accept_id)
ORDER BY reviewer_id, assign_id, assign_date DESC;


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________
Jeffery Boes <>< jboes@qtm.net