Re: Relation Schemata vs. Relation Variables

Jan Hidders wrote:
> JOG wrote:
> > Brian Selzer wrote:
> > > [snip]
> > > This is a contradiction. A transition constraint must either be able to
> > > pair the elements of two relations for comparison or be given a set of pairs
> > > to compare.
> >
> > Ok, what on earth is a transition constraint then? A transition occurs
> > when an 'entity' changes. In contrast a fact cannot 'change'. It is
> > either true or not, and that is that.
>
> Sets of facts can and do change, and transitional constraints restrict
> wich transitions from one set of fact to another are allowed. I don't
> see a fundamental problem here. Note btw. that they are a strict
> subclass of the restrictions that might be expressed by some kind of
> temporal logic.

True, a set of facts /can/ be replaced by a new set of facts (a relvar is clearly defined as a variable after all), and there may be some constraints held therein. However the focus here has been transition of individual tuples which are merely values.

I believe we are agreeing. but it is important not to muddy the waters - brian is concerned with entity transitions.

>
> However, I also think that your observation that the relational model
> is not the right abstraction level for a proper discussion about
> transition constraints in terms of updates on entities is spot on. For
> that you need to think about entity identity, static keys (that
> identify entities on a certain moment) and dynamic keys (that identify
> entities across time). It's here that Entity-Relationship-like data
> models (when properly formalized) provide a more convenient terminology
> than the relational model.

agreed.

>
> -- Jan Hidders
Received on Tue Sep 05 2006 - 23:50:23 CEST