From: Manfred Koizar <mkoi-pg@aon.at>
To: Robert Treat <xzilla@users.sourceforge.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: can i make this sql query more efficiant?
Date: Fri, 04 Apr 2003 01:13:18 +0200
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On 03 Apr 2003 16:02:04 -0500, Robert Treat
<xzilla@users.sourceforge.net> wrote:
>select 
>	event, 
>	(select count(*) from baz a 
>		where level = 1 and a.event=baz.event) as ones, 
>	(select count(*) from baz a 
>		where level = 2 and a.event=baz.event) as twos, 
>	(select count(*) from baz a 
>		where level = 3 and a.event=baz.event) as threes
>from
>	 baz
>group by 
>	event;

>which is fine, but I am wondering if there is a better way to do this?
>I'd mainly like to reduce the number of subqueries involved.

SELECT event,
       SUM (CASE level WHEN 1 THEN 1 ELSE 0 END) AS ones,
       SUM (CASE level WHEN 2 THEN 1 ELSE 0 END) AS twos,
       SUM (CASE level WHEN 3 THEN 1 ELSE 0 END) AS threes
  FROM baz
 GROUP BY event;

> Another
>improvement would be to not have to explicitly query for each level,

This might be a case for a clever set returning function, but that's
not my realm.  Wait for Joe to jump in  ;-)

Servus
 Manfred