- From: Seaborne, Andy <andy.seaborne@hp.com>
- Date: Thu, 10 May 2007 10:38:45 +0100
- To: "public-rdf-dawg@w3.org >> RDF Data Access Working Group" <public-rdf-dawg@w3.org>
- CC: ogbujic@ccf.org
Seaborne, Andy wrote:
...
> I think the notation is confusing the issue here and the notation needs some
> editorial fixing.
>
> {} isn't the empty set - it's the empty graph pattern (actually, there are two
> empty patterns, a BGP with no triple patterns, and a group with nbo elements.
> Both should give the same answers - both need to be explicitly defined).
> The empty pattern is no restrictions on the variables in a soltuion and it is
> the join identity. As an empty group, the solution to the pattern is one row
> or no bindings.
>
I've used Z for the empty pattern and stated that the BGP which is the empty
set is the empty pattern. Sec 12.3 (Basic Graph Patterns) already has the
machinary for dealing with the empty BGP and notes that the solution is a
multiset with one element (the solution which has dom(mu) = empty set, which,
writing a ampping functions in set form is the empty set) and cardinality 1.
Andy
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Received on Thursday, 10 May 2007 09:41:06 UTC