- From: Lee Feigenbaum <lee@thefigtrees.net>
- Date: Wed, 08 Jul 2009 12:36:50 -0400
- To: Simon Schenk <sschenk@uni-koblenz.de>
- CC: "public-rdf-dawg@w3.org" <public-rdf-dawg@w3.org>
Simon Schenk wrote: > Hi, > > I've been fiddling with algebra expressions for MINUS and I am wondering > whether the following capture the semantics of antijoin with restriction > to shared variables: > > A \ (A |X| [Projection to vars in A] B) > > If A and B do not share a variable, the projection results in a relation > with zero colums. I am not sure, whether this is even defined. We use it already in SPARQL. The solution of an empty pattern {} is a solution set with a single solution which binds no variables. I think your definition seems ok ... Andy pointd out to me off list that we already have a similar/equivalent definition to fall back on in SPARQL - Diff(...) http://www.w3.org/TR/rdf-sparql-query/#sparqlAlgebra . > Does anyone of the relational algebra gurus out there have a clue? Not I. Lee > Cheers, > Simon > >
Received on Wednesday, 8 July 2009 16:37:51 UTC