Re: 4NF is Where It Is At! [WAS Re: 1:1 relationships]
Date: Tue, 6 Feb 2001 22:32:07 -0500
Message-ID: <3a80c3c7_2_at_news1.prserv.net>
Jan Hidders wrote:
> Barry wrote:
> > Jan Hidders wrote:
> > > Barry wrote:
> > > > Jan Hidders wrote:
> > > > > Barry wrote:
> > > > > >
> > > > > > The classic example used to be
> > > > > > lives_at( person ) -> current_address
> > > > > > and
> > > > > > lives_at( person ) -> current_home_ph#
> > > > > > but
> > > > > > lives_at( person ) -/-> current_salary
> > > >
> > > > I'd intended {person} as the candidate key, and (unstated)
> > > >
> > > > earns( person ) -> current_salary
> > >
> > > [...] So, if earns( person ) current_salary holds then your example
> > > -> has the candidate key {person} and is therefore already in 4NF
> >
> > Doesn't having current_address and current_salary violate your
> > definition of 4NF given earlier in the thread as:
> >
> > 4NF: A relation is in 4NF if it holds for every
> > non-trivial multi-valued dependency X->>Y
> > that X is a candidate key
> >
> > I'm thinking here that multiple people from different addresses might
> > have the same current_salary ... and I never said :-) that
> > co-residents could *not* work at the same company earning different
> > salaries. So now I have current_address ->> current_salary *but*
> > current_address is *not* a Candidate Key. And a case could also be
> > made for the reverse.
>
> Yes, but these are all not multi-valued dependencies. You seem to
> think that X->>Y means that there can be multiple Ys associated with an X,
> but that is not the correct definition. The correct definition says
> that with every X there should be a set of Ys associated that is
> *independent* of the other attributes (i.e. not in X or Y).
So does that mean that each of { person } ->> { current_address } and {
person } ->> { current_salary } are independent MVDs since we've already
said that there is no connection between current_address values and
current_salary value?
Barry J.