Re: ?? Functional Dependency Question ??
From: paul c <toledobythesea_at_oohay.ac>
Date: Tue, 21 Oct 2008 16:45:01 GMT
Message-ID: <hGnLk.2943$fF3.315_at_edtnps83>
>
> What’s the problem? If there are no attributes then the only FD we
> can state is
>
> {} -> {}
>
> which is an example of a trivial FD (because rhs is a subset of the
> lhs). In the propositional calculus this maps to
>
> true -> true.
>
> The empty set of attributes (union identity) maps to true (conjunctive
> identity).
>
Date: Tue, 21 Oct 2008 16:45:01 GMT
Message-ID: <hGnLk.2943$fF3.315_at_edtnps83>
David BL wrote:
> 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).
>
Okay, but isn't this changing the original mapping which was from VALUES of attributes? Received on Tue Oct 21 2008 - 18:45:01 CEST