Re: Relation subset operators
Date: Fri, 5 Jun 2009 04:14:22 -0700 (PDT)
On 5 juin, 03:16, Gene Wirchenko <ge..._at_ocis.net> wrote:
> cim..._at_hotmail.com wrote:
> >> Why do you bring in relation subtype (and what is it, actually)?
> >A relation R1 de facto contitutes a type. A relation R2 using R1 as a
> >domain of possible values (each relation value being a possible value
> >for R2) and before applying R2 specific constraints makes R2 a subtype
> >of R1. One could view a subtype as a declaratively constrained subset
> >of tuples or alternatively as specialized subset of tuples. Since all
> >tuples in R2 necessarily belong to R2, performing aggregation between
> >R1 and R2 does make sense. For instance consider INT as a relation
> >whose body includes all integers and ODD_NUMBERS as a relation that
> >derives from INT. Nothing prevents to my knowledge performing set
> >operations between INT and ODD_NUMBERS even though they are not
> >strictly speaking of the same type.
> Sure they are. For all x IN ODD_NUMBER, x IN INT.
The fact that all elements of a specific type do belong to another type does not mean their typing is the same. Think about this analogy: ALL ORANGES are also FRUITS. But the question whether all elements of ORANGES are more FRUITS than ORANGES is a matter of perspective. As far as RM is concerned, stronger typing seems more reasonnable. Received on Fri Jun 05 2009 - 13:14:22 CEST