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Re: Comments on aggregation in SPARQL 1.1

From: Steve Harris <steve.harris@garlik.com>
Date: Wed, 18 May 2011 12:06:23 +0100
Cc: public-rdf-dawg-comments@w3.org
Message-Id: <622151C5-8885-4A77-9D4B-0941120CD8C4@garlik.com>
To: Graham Klyne <graham.klyne@zoo.ox.ac.uk>
On 2011-05-18, at 08:19, Graham Klyne wrote:

> For the purposes of your formal process, please treat this as acknowledgement of satisfactory resolution.
> ...
> Issue 2 is certainly resolved.
> For issue 1, I think the test cases probably help out, but they're not so easy to navigate for casual enquiry.  Is there a human-readable version of the test case manifest?  It seems much of the data is there from which one could be generated automatically.

Not representing the group:

The documents are for the purpose of informing development of systems. Adding sufficient text to make them readable for casual enquiry would make them unfeasibly large. That's really the job of reference texts, and tutorials. Admittedly these don't yet exist for SPARQL 1.1, as it's not yet a Recommendation, but I'm sure they will in due course.


> Steve Harris wrote:
>> In response to http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2010Sep/0000.html
>> > 1. Editorial
>> >
>> > I'm finding the section on aggregates is quite hard to follow.
>> >
>> > More examples, especially for GROUP_CONCAT, might make it easier
>> > to understand the link between the algebra and its practical consequences
>> > in SPARQL queries.
>> There is a balance between providing examples, and making the document too long.
>> The explanatory section on aggregates has been expanded, and there are now some testcases for aggregates http://www.w3.org/2009/sparql/docs/tests/data-sparql11/aggregates/ - which hopefully illustrate the behaviour.
>> > Also, I can't find any indication of the interaction between GROUP BY and ORDER
>> > BY, where the ORDER BY specifies a variable not named in the GROUP BY: is this
>> > allowed? If so, what effect does it have (e.g. on GROUP_CONCAT)?
>> Aggregate operations are performed on a multiset, so order is not preserved.
>> The group discussed the possibility of adding this feature but decided to postpone it to a future working group.
>> Please note: “ORDER BY after a GROUP BY must only use variables exposed by the GROUP.  Use of non-grouping variable outside the GROUP BY is not legal and a required syntax error.”
>> For example:
>>   SELECT ?book ?title
>>   WHERE {
>>     ?book dc:title ?title }
>>   GROUP BY ?book
>>   ORDER BY ?title
>>   ==> error.
>> > 2. A Nice-to-have feature
>> We understand that you have now seen another solution to this issue, see http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2010Oct/0001.html.
>> Could you please respond to this message saying whether this comment has been answered to your satisfaction.
>> Regards,
>>   Steve, on behalf of the SPARQL Working Group

Steve Harris, CTO, Garlik Limited
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Received on Wednesday, 18 May 2011 11:06:53 UTC

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