Re: Newbie question about db normalization theory: redundant keys OK?

From: David Cressey <>
Date: Sat, 15 Dec 2007 11:21:32 GMT
Message-ID: <0NO8j.259$1R1.92_at_trndny02>

"David Portas" <> wrote in message
> On Dec 14, 7:49 pm, "David Cressey" <> wrote:
> >
> >
> > Someone else questioned my categorization of DKNF as "final". What I
> > was this: there are no normal forms more restrictive than DKNF. This
> > been proven, although I have not seen the proof, and maybe wouldn't be
> > able to follow it if I did. The other participant pointed out that
> > table decomposition is possible. That's true, but that doesn't mean
> > the resulting schema is of a higher normal form.
> >
> Define "more restrictive". My interpretation is that you think a
> schema in DKNF must also be in 6NF, 5NF, ... etc. This is false
> though. Example:
> R{a*,b,c}
> where a* is the only candidate key.
> Given the dependencies {a}->{b} and {a}->{c} relation R IS in DKNF but
> NOT in 6NF. Does this mean 6NF is "more restrictive"? It is true to
> say that 6NF is not a proper subset of DKNF because there are some 6NF
> schemas that are not in DKNF.

As I already said, 6NF baffles me. Why is the relation above not in 6NF?

> --
> David Portas
Received on Sat Dec 15 2007 - 12:21:32 CET

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