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Algebra expression for MINUS

From: Simon Schenk <sschenk@uni-koblenz.de>
Date: Wed, 08 Jul 2009 16:31:59 +0000
To: "public-rdf-dawg@w3.org" <public-rdf-dawg@w3.org>
Message-Id: <1247070719.18947.31.camel@tweety>
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. 

Does anyone of the relational algebra gurus out there have a clue?

Cheers,
Simon


-- 
Simon Schenk | ISWeb | Uni Koblenz
http://isweb.uni-koblenz.de
http://www.uni-koblenz.de/~sschenk
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Received on Wednesday, 8 July 2009 16:32:40 GMT

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