Re: Relation Schemata vs. Relation Variables

"Jan Hidders" <hidders_at_gmail.com> wrote in message news:1157457516.222077.154380_at_b28g2000cwb.googlegroups.com...
>
> 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.
>

I don't understand what you mean. Are you saying that transition constraints can be expressed as state constraints?

> 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
>
Received on Wed Sep 06 2006 - 07:11:27 CEST