Algebra expression for MINUS

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


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Simon Schenk | ISWeb | Uni Koblenz
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