Re: ?? Functional Dependency Question ??
From: paul c <toledobythesea_at_oohay.ac>
Date: Tue, 21 Oct 2008 17:20:07 GMT
Message-ID: <bboLk.2947$fF3.1778_at_edtnps83>
>
> Regardless, it is a value. It can either exist in the relation or not.
> We can test it for equality. What else does one need to know about it?
Date: Tue, 21 Oct 2008 17:20:07 GMT
Message-ID: <bboLk.2947$fF3.1778_at_edtnps83>
Bob Badour wrote:
> paul c wrote:
>
>> Bob Badour 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?
I can see that the tuple in DEE must have a value, the empty set I presume. But when I think of an empty set of <A,T,V> triples, ie., {}, I'm darned if I can imagine a mental device for preserving a V, concrete or not, that corresponds with an A that isn't present. That's what I was quibbling about in David B's interpretation.
(Maybe I shouldn't drift the topic, but I'm guessing there's no problem with DUM! I guess empty relations never have any problem satisfying relational closure as long as their constraints aren't effectively negations, eg., "IS_EMPTY".) Received on Tue Oct 21 2008 - 19:20:07 CEST