Re: circular relationships ok?

"vc" <boston103_at_hotmail.com> wrote in message news:1141397694.943802.23190_at_t39g2000cwt.googlegroups.com...
>
> Brian Selzer wrote:
>>[...]
>> if a and b are values
>> for A and B respectively in a tuple of R, a implies b and b implies a,
>> therefore, a iff b.
>
> That does not make any obvious sense. What do you mean by '[value] a
> implies [value] b' ? Are you considering only the Boolean domain ?
>

A key is a set of attribute values, or facts. A set of facts is a proposition in conjuctive normal form. Therefore, a functional dependency is essentially a logical implication. For example, assume that a tuple in a relation with the attributes A, B, and C has values a, b, and c respectively. Then the following statements are true: A has value a. B has value b. C has value c. If {A, B} is a candidate key, then the statement "A has value a, and B has value b." implies the statement "C has value c." because by definition, the value of a candidate key determines all other values in a tuple.

> Neither does that:
>
> [...]
>>...for any set of values, a, b and c for A, B, and C respectively,
>>
>> a implies b, b implies c and c implies a.
>>
>
> [...]
>
Received on Sat Mar 04 2006 - 11:40:05 CET