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Home -> Community -> Usenet -> comp.databases.theory -> Re: 4NF is Where It Is At! [WAS Re: 1:1 relationships]
Jan Hidders wrote:
> Barry wrote:
> > ...
> > Been a *long* time since I've had an opportunity to discuss this, so
> > I'm doubtless in need of a refresher. But the 4NF formulation I
> > remember is something like:
> >
> > For tuple (k, a, b) where k is the key, a and b are non key attributes,
> > and
> > f( k ) -> a, then f( k ) -> b for 4NF to be satisfied. If g( k ) -> b,
> > then the tuple needs to be decomposed into (k, a) and (k, b).
>
> ...
>
> > The classic example used to be
> > lives_at( person ) -> current_address
> > and
> > lives_at( person ) -> current_home_ph#
> > but
> > lives_at( person ) -/-> current_salary
>
> Yes, this is a correct example. Let me explain how the definition of 4NF
> deals with this. We are talking about the following relation
>
> R(Person, Curr_addr, Curr_home_ph, Curr_sal)
>
> It follows from the dependencies you gave that there is only one
> candidate key for this relation: {Person, Curr_sal}. ...
I'd intended {person} as the candidate key, and (unstated)
earns( person ) -> current_salary
as an example of "g(k)->b" justifying, in my 4NF "formulation", current_salary being split from the other attributes. And the prefixing of "current" on each was meant to imply there is only a single value in each case for a given candidate key.
Barry J. Received on Sun Jan 28 2001 - 10:55:47 CST