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Re: Blank node treatment is unclear [resent]

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Thu, 01 Jun 2006 10:40:09 +0100
Message-ID: <447EB5F9.6020807@hp.com>
To: Simon Gibbs <simon.gibbs@cantorva.com>
CC: public-rdf-dawg-comments@w3.org



Simon Gibbs wrote:
> Hello
> 
> Following on from a chat on #swig I have a test case for discussion
> purposes. See http://cantorva.com/2006NS/Section0214BlankNodesTestCase.rdf
> for test data and commentry.
> 
> I've implemented a SPARQL engine which implements blank nodes per
> section 2.8.3 which gives blank node labels query scope. Section 2.5.4
> seems to contradict this giving labels a scope specific to the basic
> graph pattern. One contributor on #swig understood 2.5.4 to be the case.
> 
> See notes in the linked RDF/XML file for the differences I estimate
> this will cause in the result sets. Essentially, given the linked
> data, ?category is bound to a value less frequently under the regime
> given by 2.8.3 since all patterns must match the same subject node
> rather than matching only on the variable ?mailbox.
> 
> PREFIX ex: <http://example.org/exNS/>
> SELECT ?mailbox ?category
> WHERE
> {
>     _:a     ex:sender ?mailbox ;
>             ex:recipient <mailto:alice@example.com> .
>     OPTIONAL {
>         _:a ex:sender ?mailbox;
>             ex:category ?category .
>     }
> }
> 
> The wording in the spec that seems to be at the root of this confusion is:
> 
> 2.5.4: "the scope of the blank node label being the basic graph pattern"
> 2.8.3: "Blank nodes have labels which are scoped to the query."
> 
> both from http://www.w3.org/TR/rdf-sparql-query/
> 
> My questions are, is this a conflict or am I missing something? If so,
> what is that? Otherwise, can the WG please clarify the conflict?

No - you're not missing anything.

Blank node labels are scoped to the basic graph pattern as per 2.5.4.  2.8.3
is wrong.

This clarification/change happened very late in the working group discussions
and 2.8.3 didn't get changed.  I have corrected it in the editors' working draft.

http://www.w3.org/2001/sw/DataAccess/rq23/#blankNodes

I also changed the title of 2.8.3 to "Blank Nodes in Queries" and
adjusted the TOC.


So, your example query isn't going to treat _:a as the same in the fixed and
optional basic graph patterns - that would need a "?" variable.

	Thanks for the comment,
	Andy

> 
> Finally, thanks to all for the all of great work going into SPARQL.
> 
> Simon Gibbs
> Cantorva
> 
> PS please forgive possible duplicate
Received on Thursday, 1 June 2006 09:40:29 GMT

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