Re: Relation Schemata vs. Relation Variables

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.

-- 
David Portas