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Re: XQuery and relational databases...

From: Jim Melton <jim.melton@acm.org>
Date: Fri, 07 Mar 2003 08:54:02 -0700
Message-Id: <>
To: Nitin Mangtani <nitinm@bea.com>
Cc: Michael Burbidge <mburbidg@adobe.com>, www-ql@w3.org


Better late than never, right?

I believe that the reason XQuery does not have a grouping operator in 
version 1 has at least 2 components.  First, the perception was (and is) 
that grouping is sufficiently complex that we shouldn't delay progress on 
the rest of the language just to permit adding grouping.  Second, there 
exists a number of people (I will, with my tongue only partly in my cheek, 
refer to them as "doc-heads") who believe that grouping is an order of 
relatively low importance.

It has been asserted that the relational grouping operator (GROUP BY) is 
not appropriate for use in an XML query language.  I admit that I don't 
fully understand the arguments (yet), but I have a sort of intuitive 
understanding of at least some of the issues.  The most important one is, 
of course, that the relational GROUP BY operates strictly on a virtual 
table (a regular, rectangular structure with every cell occupied by 
*something*, even if the null value), while an XQuery grouping operator 
would have to operate on semi-structured tree structures.  The challenges 
of the latter clear exceed those of the former, even if some lessons of the 
former can be learned and used in designing the latter.

Jonathan Robie (and, to be fair, others) did a considerable amount of work 
to try to persuade the WG of the necessity of having a grouping 
capability.  Recently, Jonathan made a valiant effort to get a grouping 
capability into the language...sort of a last-minute attempt.  Don 
Chamberlin wrote a message to Jonathan, me, and at least a few others 
(perhaps to the entire Query WG?) that illustrated a grouping capability 
that is quite useful in certain situations (e.g., grouping by individual 
elements and not multiple elements.  This approach is probably "good 
enough" for version 1, allowing us to consider a more complete grouping 
facility in a future version of the XQuery specification.

I agree with you that a direct grouping operator makes life easier for 
users.  (It might or might not make it easier for vendors, depending on how 
much attention is paid to optimization of grouping.)  But I think that it 
really is too late in the process to add such an operator now.

Hope this helps,

At 21:03 2003-03-06 -0800 Thursday, Nitin Mangtani wrote:
>   Sorry for jumping on this tread late, why  "Group By" did not make into 
> XQuery spec. I understand one can use combination of distinct and 
> aggregate function to simulate a Group  By. But having a direct Group By 
> makes life much easier for everyone (Customers, Vendors, you? and me;) ).

Jim Melton --- Editor of ISO/IEC 9075-* (SQL)     Phone: +1.801.942.0144
Oracle Corporation            Oracle Email: mailto:jim.melton@oracle.com
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Sandy, UT 84093-1063              Personal email: mailto:jim@melton.name
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=  Facts are facts.  However, any opinions expressed are the opinions  =
=  only of myself and may or may not reflect the opinions of anybody   =
=  else with whom I may or may not have discussed the issues at hand.  =
Received on Friday, 7 March 2003 10:56:34 UTC

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