From: Michael Glaesemann <grzm@seespotcode.net>
To: Richard Huxton <dev@archonet.com>
Cc: Walter Cruz <walter.php@gmail.com>
Cc: pgsql-sql@postgresql.org
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
Message-ID: <57FD2B24-DB43-4EF7-92D1-3C467641AF9D@seespotcode.net> (raw)
In-Reply-To: <45D46196.2090003@archonet.com>
References: <32cabba0702150319v427d5a25s9a6c7f851576baa2@mail.gmail.com>
<45D46196.2090003@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
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To: pgsql-sql@postgresql.org
Cc: grzm@seespotcode.net, dev@archonet.com, walter.php@gmail.com
Subject: Re: "for SELECT DISTINCT, ORDER BY expressions must appear in select list" - is that the standart or a limitation of postgresql?
In-Reply-To: <57FD2B24-DB43-4EF7-92D1-3C467641AF9D@seespotcode.net>
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