Date: Sun, 9 Feb 2003 17:12:04 -0600
From: "Ross J. Reedstrom" <reedstrm@rice.edu>
To: "A.M." <agentm@cmu.edu>
Cc: pgsql-sql@postgresql.org
Subject: Re: order by date desc but NULLs last
Message-ID: <20030209231204.GB11550@wallace.ece.rice.edu>
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	"A.M." <agentm@cmu.edu>, pgsql-sql@postgresql.org
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On Sun, Feb 09, 2003 at 05:29:29PM -0500, A.M. wrote:
> I have a simple query that sorts by descending date but the NULL dates 
> show up first. Is there a way I can sort so they come last without 
> sorting ascending?
> 
> SELECT submittime,score,gradedtime FROM student_gradedmaterial WHERE 
> gradedmaterialid=3 and studentid=102 order by gradedtime desc;
> 
>      submittime      | score |         gradedtime
> ---------------------+-------+----------------------------
>  2003-01-30 22:56:38 |       |
>  2003-01-31 03:42:29 |    99 | 2003-02-06 14:21:43.043587
> 
> but what I want is all the graded items first in gradedtime desc and 
> NULL afterwards. I do need to keep the NULL score rows. (So I get the 
> latest submitted grade for the assignment but also any ungraded 
> submission information.)


You need to ORDER BY a _function_ of the gradedtime column, substituting
an extreme value for NULL. Try this:

 SELECT submittime,score,gradedtime FROM student_gradedmaterial
 WHERE gradedmaterialid=3 and studentid=102 order by
 coalesce(gradedtime,'-infinity') desc;

Ross