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	Thu, 23 May 2002 23:17:34 +0500
Subject: Re: DISTINCT and ORDER BY issue
From: Hannu Krosing <hannu@tm.ee>
To: terry@greatgulfhomes.com
Cc: "Postgres (E-mail)" <pgsql-docs@postgresql.org>
In-Reply-To: <001401c20268$fdff0c40$2766f30a@development.greatgulfhomes.com>
References: <001401c20268$fdff0c40$2766f30a@development.greatgulfhomes.com>
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Date: 23 May 2002 23:17:32 +0500
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On Thu, 2002-05-23 at 19:49, terry@greatgulfhomes.com wrote:
> I know what I am doing wrong:  The order by needs to match the DISTINCT
> fields.
> 
> Here's what I am doing:  Get the 10 most recent applications user has used
> to put in their "recently used" menu.
> 
> So I need to ORDER BY user_app_access_log.access_stamp DESC
> 
> But I want each record distinct, eg if the last 5 hits were all for the same
> app, I want 1 record returned for that.  So if I put the access_stamp field
> in the DISTINCT field set, then I go back to getting repeats of apps, hence
> not 10 unique app names...
> 
> I wonder if there is a way to use an aggregate function to just return the
> first value, (ignoring the rest of the group), and use the group by clause
> on the app_name?
> 
> Any ideas is appreciated.
> 
> Here is my query/error message:
> Error while executing the query; ERROR: For SELECT DISTINCT, ORDER BY
> expressions must appear in target list
> 
> 
> SELECT DISTINCT applications.app_name, applications.app_long_name,
> applications.app_url, user_access.in_new_window
> FROM applications, user_access, user_app_use_log
> WHERE user_app_use_log.user_id = '1'
> AND user_app_use_log.app_name != 'index'
> AND user_access.app_name = user_app_use_log.app_name
> AND user_access.user_id = user_app_use_log.user_id
> AND user_access.division_id = 'GGH'
> AND applications.app_name = user_access.app_name
> ORDER BY user_app_use_log.access_stamp DESC
> LIMIT 10"

You can try either

SELECT DISTINCT app_name,app_long_name,app_url,in_new_window
FROM (
  SELECT
     user_app_use_log.access_stamp,
     applications.app_name,
     applications.app_long_name,
     applications.app_url,
     user_access.in_new_window
  FROM applications, user_access, user_app_use_log
  WHERE user_app_use_log.user_id = '1'
  AND user_app_use_log.app_name != 'index'
  AND user_access.app_name = user_app_use_log.app_name
  AND user_access.user_id = user_app_use_log.user_id
  AND user_access.division_id = 'GGH'
  AND applications.app_name = user_access.app_name
  ORDER BY user_app_use_log.access_stamp DESC
) t
LIMIT 10
;

or 

SELECT
  applications.app_name,
  applications.app_long_name,
  applications.app_url,
  user_access.in_new_window,
  max(access_stamp) as max_access_stamp
FROM applications, user_access, user_app_use_log
WHERE user_app_use_log.user_id = '1'
AND user_app_use_log.app_name != 'index'
AND user_access.app_name = user_app_use_log.app_name
AND user_access.user_id = user_app_use_log.user_id
AND user_access.division_id = 'GGH'
AND applications.app_name = user_access.app_name
GROUP BY 1,2,3,4
ORDER BY max_access_stamp DESC
LIMIT 10
;

and see which one is more effective

---------------
Hannu