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Re: Restructure definition of Basic Graph Pattern and pattern match (sec 2.4)

From: Jeen Broekstra <jeen@aduna.biz>
Date: Fri, 10 Jun 2005 10:36:45 +0200
Message-ID: <42A9511D.5030603@aduna.biz>
To: Dan Connolly <connolly@w3.org>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>

Dan Connolly wrote:
> On Thu, 2005-06-09 at 14:54 +0200, Jeen Broekstra wrote:

[snip]

>>Suggested definitions:
>>
>>   Definition: *Basic Graph Pattern*
>>
>>   A /basic graph pattern/ is a set of Triple Patterns.
>>
>>   Definition: *Graph Pattern Match*
>>
>>   A graph pattern /matches/ on a graph G with pattern solution S iff
>>   S(GP) is a non-empty subgraph of G.
> 
> 
> Yes, but... why non-empty? Consider:
> 
>  SELECT * where {}.
> 
> In the case of the empty solution (i.e. empty set of bindings),
> S(GP) is just GP, which is empty. We want that to be a match,
> right? i.e. ASK where {} always gives yes, right?

To clarify: I included the non-empty bit because I thought it would 
otherwise regress to the empty set being a valid match for _any_ graph 
pattern.

I just had a look again however, and this is not the case. (My error 
was that I thought S might map to an empty set; I'd misread something 
in the definition of the mapping. Mea culpa).


Jeen
-- 
Jeen Broekstra          Aduna BV
Knowledge Engineer      Julianaplein 14b, 3817 CS Amersfoort
http://aduna.biz        The Netherlands
tel. +31 33 46599877
Received on Friday, 10 June 2005 08:38:02 GMT

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