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Home -> Community -> Usenet -> comp.databases.theory -> Re: Little question for RDM theoristes
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 - 08:49:41 CDT