Re: Relation Schemata vs. Relation Variables

From: Brian Selzer <brian_at_selzer-software.com>
Date: Tue, 05 Sep 2006 04:23:15 GMT
Message-ID: <TY6Lg.1301$MF1.543_at_newssvr25.news.prodigy.net>


"JOG" <jog_at_cs.nott.ac.uk> wrote in message news:1157414534.666540.38270_at_i42g2000cwa.googlegroups.com...
> 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.
>

By the same token, what on earth is a functional dependency, or an inclusion dependency--or a key, for that matter?

Oh, and isn't a database just one big fact? Are you saying that databases shouldn't change?

> 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.
>

So, what you're saying is that we should toss all constraints that are semantic in nature?

> 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.
>

What assumption is that? Received on Tue Sep 05 2006 - 06:23:15 CEST

Original text of this message