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Re: Algebra expression for MINUS

From: Lee Feigenbaum <lee@thefigtrees.net>
Date: Wed, 08 Jul 2009 12:36:50 -0400
Message-ID: <4A54CB22.5080601@thefigtrees.net>
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 GMT

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