Re: Relation Schemata vs. Relation Variables

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.

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.  

• Jan Hidders