Re: MV Keys

From: David Cressey <dcressey_at_verizon.net>
Date: Fri, 03 Mar 2006 20:53:52 GMT
Message-ID: <A12Of.24$FN1.14@trndny04>

"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message news:MPG.1e713f6a4e8227b1989779_at_news.ntnu.no...
> In article <1141311212.487197.167020_at_t39g2000cwt.googlegroups.com>,
> jog_at_cs.nott.ac.uk says...
> > What is the advantage if any of this functional mapping (one-one, where
> > an attribute maps to a single value) over the more general
> > mathematical-relational mapping (where an attribute may map to many
> > values)?
> >
> > Is this just so the resulting constructs can be visualised as nice neat
> > regular tables? I am yet to hear any other reasons, but I don't
> > preclude their existence. Does it unduly affect the algebra somehow?
> > Anyone?
>
> It depends on precisely what you mean. If you envision a relational-like
> model where each "cell" (speaking loosely) can contain several values,
> several questions arise (to me, at least):
>
> Join is crucial, and depends on equality between "cells". But with
> multiple values, how do you define equality?
>
> Is (apples, pears) equal to (pears, apples)?

An alternative example would be:
Is (onions, mushrooms) equal to (mushrooms, onions)? I bleieve I've aske this question several times. Received on Fri Mar 03 2006 - 14:53:52 CST