Re: ?? Functional Dependency Question ??

On Oct 21, 11:54 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> David BL wrote:
>
> ...
>
> > Consider that in the FD world symbol X represents a set of attributes
> > from some relation R. Let some tuple of R be given. Then as a
> > proposition we interpret X as implying that we are given or can deduce
> > (for the given tuple) the values of all the attributes associated with
> > X. This interpretation makes it obvious that unions of attributes
> > map to logical conjunctions, and that an FD maps to a logical
> > implication.
>
> Thanks, but how does that interpretation work when R has no attributes?

What’s the problem? If there are no attributes then the only FD we can state is

    {} -> {}

which is an example of a trivial FD (because rhs is a subset of the lhs). In the propositional calculus this maps to

   true -> true.

The empty set of attributes (union identity) maps to true (conjunctive identity). Received on Tue Oct 21 2008 - 18:15:55 CEST