# Re: Little question for RDM theoristes

Date: 16 Jun 2006 06:49:41 -0700

Message-ID: <1150465781.863543.63220_at_p79g2000cwp.googlegroups.com>

Sorry made a TYPO...

I wrote...

a relation is not an unordered set of values at logical level....
I meant...

a relation is an unordered set of values at logical level....

First would be obviously wrong....

Cimode wrote:

> I am aware of that definition and I do not quite agree with it because

*> it leads to confusion...
**>
**> CJ Date is well known for using ambivalent terminology for
**> vulgarization purposes towards SQL audiences...I personally admire his
**> patience with SQL people given the bad it did to RM on the past 20
**> years...
**> For instance he uses Tables for relvar projection (which typically
**> creates confusion with SQL tables) while Pascal prefers R-Table...
**>
**> I am more a follower of the FP, McGoveran approach who advocate a
**> tighter commitment to terminilogy ...
**>
**> A relation is BOTH a relvar which represent the abstract structure of
**> the relvar and the relvalues which represents its matter at a specific
**> point in time.
**>
**> Think about the implication of stating that a relation1 = (set of
**> relvalues)1 (relvalues drawn from relation1 domain1 of possible values)
**>
**> if relation1 = (set of relvalues)1 of domain1 and relation2 = (set of
**> relvalues)1 of domain1
**>
**> You can conclude that relation1 = relation2 IF AND ONLY IF you assume
**> all values have the same location in a multidimensional
**> representation...Admitting such axiom would lead to accept that all
**> relvalues in a specific relvar are always located at the same position
**> at any time...Think about the implications: All projections would be
**> then necessarily ordered sets of values...a relation is not an
**> unordered set of values at logical level....This would be totally silly
**>
**> Therefore defining a relation only through its values is unsufficient
**> to allow to both represent faithfully and operate the relation...But
**> SQL people need this kind of confusion to make sense of what a relvar
**> is...
**>
**>
**> Erwin wrote:
**> > > Your question implies relations = relvalues...which if I follow this
**> > > false premise reasonning would lead to relations that have similar
**> > > relvalues being equal which is totally false...2 relvar with same
**> > > relvalues are NOT necessarily equal.
**> >
**> > TTM Chapter 4, RM prescription 10 :
**> >
**> > "A relation value (relation for short) ..."
**> >
**> > Therefore at least to Chris Date, 'relations=relvalues' is most
**> > certainly true. I'd say that's a strong indication of just how much
**> > "false premise" there is within.
**> >
**> > > This question is totally irrelevant if you consider a relation as being
**> > > equal to a relvalue...
**> >
**> > This question is not irrelevant at all since the heading is regarded as
**> > the definition of the applicable relation type. And for values to be
**> > equal, they must most certainly be of the exact same type, inheritance
**> > issues notwithstanding of course.
*

Received on Fri Jun 16 2006 - 15:49:41 CEST