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Cc: Walter Cruz <walter.php@gmail.com>,
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From: Michael Glaesemann <grzm@seespotcode.net>
Subject: Re: "for SELECT DISTINCT,
	ORDER BY expressions must appear in select list" - is that the
	standart or a limitation of postgresql?
Date: Thu, 15 Feb 2007 23:04:04 +0900
To: Richard Huxton <dev@archonet.com>


On Feb 15, 2007, at 22:35 , Richard Huxton wrote:

> Walter Cruz wrote:
>> SELECT distinct name from test order by number
>> (well, I think that que query doesn't make any sense, but raises  
>> the error :) )
>> The error: ERROR:  for SELECT DISTINCT, ORDER BY expressions must
>> appear in select list is due to a standart implementarion or a design
>> decision of postgres?
>
> I think ORDER BY is defined to take place after DISTINCT, which  
> means there is no meaningful "number" for it to order by. You could  
> arbitrarily choose the first number encountered, but I can't see  
> what sense it would make to order by them.

I believe the reason is that DISTINCT depends on a sort to determine  
uniqueness (distinctness), so it's a implementation detail that if  
you're going to include an ORDER BY, you also need to include the  
same columns in the ORDER BY in the DISTINCT clause. Though I suspect  
Richard is right that ORDER BY takes place after DISTINCT. (My  
cursory attempt at parsing the SQL 2003 draft failed me.)

On further thought, I bet

SELECT DISTINCT name
FROM test
ORDER BY name, number

fails with a different error, one directly supporting Richard's  
conclusion.

Michael Glaesemann
grzm seespotcode net