# Re: Relation subset operators

Date: Sat, 06 Jun 2009 19:31:23 GMT

Message-ID: <fuzWl.30895$PH1.15504_at_edtnps82>

cimode_at_hotmail.com wrote:

...

*> I would go further than that into saying that previous work has only
*

> clarified side effects of relation operations. And a lot of it missed

*> the mark into expressing properties of operations that can not be
**> expressed without proper quantifiers. For instance, does the empty
**> set has the same role place in relational theory, than the zero would
**> have in traditional algebra. Up till, such questions have not been
**> answered and these claims have neither been properly demonstrated nor
**> they have been properly evaluated. ...
*

In relational algebra, I know of only two contexts for the empty set, one is the empty heading/attribute set, the other is the empty relation/tuple set. The first has two values, the second can have many values. Neither operates like arithmetical zero, for example division by the empty set is defined whereas it is undefined for zero. I'm not sure what "questions" remain unanswered, as far as I know both empty set contexts are defined and both function as identities that give relational closure, unlike arithmetic. Received on Sat Jun 06 2009 - 21:31:23 CEST