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Re: How can I get the last element out of GROUP BY sets?

From: Robert Creager <Robert_Creager(at)LogicalChaos(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PGSQL <pgsql-sql(at)postgresql(dot)org>,PGSQL <pgsql-sql(at)postgresql(dot)org>
Subject: Re: How can I get the last element out of GROUP BY sets?
Date: 2004-01-19 17:38:53
Message-ID: 20040119103853.01c87b83.Robert_Creager@LogicalChaos.org (view raw or flat)
Thread:
Lists: pgsql-sql
When grilled further on (Mon, 19 Jan 2004 00:44:30 -0500),
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> confessed:

> Robert Creager <Robert_Creager(at)LogicalChaos(dot)org> writes:
> > ... one piece of data I need is the last value for each GROUP BY
> > period.  Alas, I cannot figure out how to do this.
> 
> SELECT DISTINCT ON (rather than GROUP BY) could get this done for you.
> 

I had my whine all ready as to how I still couldn't figure it out, when I
figured it out:

...
SELECT p.period, etday
FROM (SELECT DISTINCT ON ( period )
         date_trunc( 'hour', "when" ) AS period,
         etday
      FROM readings
      ORDER BY period, "when" DESC) AS p
...

Thanks for the tip.

Cheers,
Rob

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