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From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: "pgsql-docs@postgresql.org" <pgsql-docs@postgresql.org>
Subject: Re: Is this example regarding aggregates sourced by subquery correct?
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Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
	message dated "Fri, 20 May 2016 16:08:27 -0400"
Date: Fri, 20 May 2016 17:07:28 -0400
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> http://www.postgresql.org/docs/9.5/static/functions-aggregate.html
> """
> SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
> But this syntax is not allowed in the SQL standard, and is not portable to
> other database systems.
> """

> The example seems expressly permitted by the standard and other database
> systems.

The example is illegal in SQL:2003 and before; they did not allow ORDER BY
in a <query expression> until SQL:2008.  Even in newer spec versions, it's
considered an optional feature (cf F850-F855).  Given that, I would be
pretty leery of claims that it's supported in all other DBMSes.  We should
perhaps back off the wording to something like "is not allowed in older
versions of the SQL standard, and may not be portable to other database
systems".

			regards, tom lane


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