Re: Lucid statement of the MV vs RM position?

Jon Heggland wrote:
> GUNF intuitively seems very simple to me; like most NFs, it's just
> formalised common sense. Consider an example of the relvar keys relvar (1):
>
> Relvar|Key
> ======+========
> R1 { A }
> R2 { B }
> R1 { C, D }
>
> ---i.e. relvar R1 has two keys, { A } and { C, D }, while R2 has but
> one, { B }. Ungrouping (unnesting) it produces the following relation (2):
>
> Relvar|Key component
> ======+=============
> R1 A
> R2 B
> R1 C
> R1 D
>
> ---and grouping it again produces this (3):
>
> Relvar|Key (wrong)
> ======+-----------
> R1 { A, C, D }
> R2 { B }
>
> (3) is different from (1), hence (1) is in GUNF, and the use of an RVA
> is justified. (Actually, Darwen's GUNF formulation is slightly
> different, but I believe my example captures its intent. The point is (I
> think) that there does not exist any relation of type (2) that when
> grouped produces (1).)

These are great examples Jon. GUNF is perhaps a missing piece in my theoretical understanding as it seems to state formally what I have been struggling with intuitively for months, when I've commented that "sets shouldn't just pop into existence from nowhere". I have been struggling with an example for a while:

A fish can be gold.
A cow can be brown.
A cow can be black and white.

Animal | Colour

Fish     { Gold }
Cow     { Brown }
Cow     { Black, White }

With the PK { Animal, Colour }. If I Ungroup I will generate:

Animal | Colour

But this is now not in GUNF because if I regroup I'll get:

Animal | Colour

Fish     { Gold }
Cow     { Brown, Black, White }

Which is of course nonsense in reference to my original predicate. Now if my understanding is correct, to model this albeit very rough information, I can now formally show that I will require an RVA. (I could use a surrogate but I would rather not materialize artificial new data just to accomodate what I have.)

Someone affirm this to me or shoot my naivite down in flames.

>
> In contrast, the Barney/Oscar relvar (1):
>
> Name |Colours
> ======+-------
> Barney { green, purple }
> Oscar { green, black }
>
> is *not* in GUNF, because there exists a relation (2)
>
> Name |Colour
> ======+======
> Barney green
> Barney purple
> Oscar green
> Oscar black
>
> that when grouped produces (1). Switching names and colours does not
> help, of course.
>
> Your supplier/parts example is probably equivalent to one of the above;
> I'm not sure which, since you don't give all the necessary details (viz.
> keys/FDs).
> --
> Jon
Received on Thu May 04 2006 - 19:45:58 CEST