Re: RA with MV attributes

JOG wrote:
> Marshall wrote:
> > Interesting post. Some comments inline.
> >
> > On Jan 16, 12:40 am, "David" <davi..._at_iinet.net.au> wrote:
> > >
> > > Definition: A relation r consists of a relation-type A(r) together
> > > with a set of tuples T(r), where each tuple is a map from each
> > > attribute a inA(r) to a subset of D(a).
> >
> > I realize it's becoming fashionable, but I still dislike the idea
> > that a relation "consists of" its type and its value, both.
> > It's type is its type and its value is its value; it doesn't
> > make sense to me otherwise. If we consider the relation
> > as being a combination of two things, we ought to be
> > able to consider those two things separately. In which
> > case we can ask the question, what is the type of a
> > relation's value? Presumably that is also its type, and
> > so we have infinite regress.
>
> I also find it an extremely uncomfortable definition. A mathematical
> relation has no "relation type", so I prefer Pascal's standpoint where
> a relation is a set of functions mapping attributes to values.

Huh? That’s what I did (except of course that I mapped attributes to sets of values to investigate the MV approach)

> It seems
> to me to appeal far more to conventional set theory and directly
> illustrates why db-attributes are necessarily unordered, again not a
> feature of mathematical relations.

Sure. Note that the join operator I defined is commutative.

[snip] Received on Thu Jan 18 2007 - 03:35:15 CET