Re: Relation Schemata vs. Relation Variables

David Portas wrote:
> Marshall wrote:
> >
> > The idea of transition constraints is inherrently non-set-theoretic,
> > because it assumes tuple-identity. Since it is possible to
> > model non-tuple-identity scenarios in the RM, there will be
> > some scenarios in which transition constraints do not make
> > sense. This is not a flaw in RM; rather it is an indication that
> > transition constraints are a lower-level albeit useful hack
> > on top of some subset of RM schemas, that are applicable
> > only some of the time.
>
> More precisely, the idea of *tuple-level* transition constraints is
> inherently non set-theoretic. Fortunately, logical operations of any
> kind at tuple-level are not possible or permitted in RM. The only
> applicable way to define a transition constraint is in set-based
> fashion at the logical level. Truly relational transition constraints
> would therefore suffer none of the problems mentioned.

Yes, exactly; thank you for pointing this out. I was not sufficiently explicit.

I haven't seen much written about transition constraints, and would be happy to read more if anyone has any good pointers. I went a-Googling a week or two ago for writings on the topic and the only interesting thing I came across was a forgotten post by me asking people about transition constaints! Ha ha!

Marshall Received on Mon Aug 21 2006 - 03:18:48 CEST