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Re: SPARQL transitive closure discussion

From: James Cheney <jcheney@inf.ed.ac.uk>
Date: Sun, 21 Mar 2010 12:01:58 +0000
Cc: "<public-xg-prov@w3.org>" <public-xg-prov@w3.org>
Message-Id: <82704BAD-3C13-4EC2-BABB-C447D853FB72@inf.ed.ac.uk>
To: Satya Sahoo <sahoo.2@wright.edu>
('binary' encoding is not supported, stored as-is)
Hi,

I'm having trouble understanding the issue under discussion in the  
linked thread.

I thought the point raised on Friday was that SPARQL lacks the ability  
to say "follow an edge zero or more times", which is needed for  
provenance queries.

As I understand it (i.e., not very well), SPARQL allows queries based  
on graph patterns, which are close to what database people call  
conjunctive queries (with some extensions, such as optional steps).   
Such queries cannot perform transitive closure queries such as  
reachability in a graph.

Does it make sense to have a query language that can take a directed  
graph and calculate the reachability relation on that graph?  This is  
conceptually no problem, and this is what transitive closure in some  
flavors of SQL allows you to do.  I see no reason why RDF would have  
to be changed to allow this in a query language such as SPARQL.  And I  
think this is what would be needed for querying provenance.

However, the discussion in the thread you linked to seems to be about  
something else:

Does it make sense to have a query language that returns graphs whose  
edges have (implicit) semantics "this edge is transitive", when the  
underlying graph model does not have such edges?  I think this is what  
Pat Hayes was arguing against in the thread; while I agree this is  
potentially confusing, it doesn't seem relevant to the kind of  
transitive queries needed for provenance.

But this is based on < 15 minutes of thinking.  So maybe I'm missing  
something.

--James


On Mar 19, 2010, at 4:11 PM, Satya Sahoo wrote:

> Hi,
> The link to discussion thread on SPARQL transitive closure: http://lists.w3.org/Archives/Public/public-swbp-wg/2005Dec/0036.html
>
> Thanks.
>
> Satya
>
> Kno.e.sis Center
> http://knoesis.wright.edu/researchers/satya



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Received on Sunday, 21 March 2010 12:02:34 GMT

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