Re: ?? Functional Dependency Question ??
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 21 Oct 2008 13:55:30 -0300
Message-ID: <48fe0995$0$5470$9a566e8b_at_news.aliant.net>
>
> I'm not sure about that. Maybe I'm being too literal in asking about
> this, but David B is talking about a mapping that involves values of
> attributes. I don't know what the value of "no attribute" is.
Date: Tue, 21 Oct 2008 13:55:30 -0300
Message-ID: <48fe0995$0$5470$9a566e8b_at_news.aliant.net>
paul c wrote:
>> paul c 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? >> >> The same way DEE works with join.
>
> I'm not sure about that. Maybe I'm being too literal in asking about
> this, but David B is talking about a mapping that involves values of
> attributes. I don't know what the value of "no attribute" is.
Regardless, it is a value. It can either exist in the relation or not. We can test it for equality. What else does one need to know about it? Received on Tue Oct 21 2008 - 18:55:30 CEST