Re: Relation Schemata vs. Relation Variables

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.

I conclude therefore that transitions are associated with entities alone, as are constraints upon them. The RM is unconcerned with 'entities' and hence is equally unconcerned with transition constraints.

This whole question is therefore moot.

> In order for the transition constraint to pair tuples, there
> must exist at least one set of identifying attributes (a key) that is
> *guaranteed* to remain constant throughout an update--any update. One way
> to do that would be to specify which key will remain constant during a
> particular change, but without tuple identifiers it's possible for all keys
> to change at the same time. Do you treat such an occurance as a
> delete...insert? Just as there may be constraints that apply only to tuples
> that are about to be removed, there may be constraints that apply only to
> tuples that are about to be added. How can the system know whether or not a
> particular transition constraint applies? Or do you just allow the change
> based on the assumption that the user knows what he's doing?

The rest of this argument is based on a confused assumption AFAICT. Received on Tue Sep 05 2006 - 02:02:14 CEST