Re: Multiple keys and transition constraints

Brian Selzer wrote:
> Given a relation schema R {A, B, C}, where A and B are each candidate keys.
>
> If the current extension is
>
> r {{A:1, B:9, C:3}
> {A:2, B:8, C:4}}
>
> and the proposed extension is
>
> r' {{A:1, B:8, C:4}
> {A:2, B:9, C:3}}
>
> is only A different? Or are both B and C different? From one perspective,
> both B and C remain constant but A is different. From another perspective,
> A remains constant but both B and C are different. If you're trying to
> specify the allowable transitions for C, which key do you use? A, B, the
> superkey, AB, or none of the above?

This requires more information to answer.

Can you tell us which attribute identifies the subject [of each proposition] in real life over the updates. Then I can determine /what/ exactly has experienced the transition.

No other questions, just that.

>
> Is the problem one of normalization? The above schema is in 5NF. Is there
> an eqivalent but "better" database schema that leaves no doubt as to which
> key to use? Some of the functional dependencies implied by the keys are:
>
> A --> B
> B --> A
> A --> C
> B --> C
> AB --> C
> A --> B --> C
> B --> A --> C
>
> Note the transitive FDs that are a result of the cyclical relationship
> between A and B.
>
> Should the attributes participating in a "mutual functional dependency,"
> that is, A <--> B (both A --> B and B --> A), be separated into another
> relation schema? For example, R above could be decomposed into S {A, B} and
> T {A, C}. One obvious advantage would be that each and every datum in the
> database would have one and only one direct, minimal and nontrivial
> determinant, so there would be no problem choosing a key when specifying a
> transition constraint. On the other hand, a circular inclusion dependency
> would need to be specified and enforced. In addition, if one of the keys in
> S were updated along with T in a multiple assignment, you still couldn't
> tell given only s, s', t and t' whether A alone is different or both B and
> C.
>
> Does the stated intent of the user matter? If a user identifies a datum by
> using the Guaranteed Access Rule and then asserts a new value, then the
> user's intent is clear and stated. The same is true if a user targets a
> restricted projection of a relation and then for each element asserts a new
> value. But even if the intent were to update only A in the example above,
> the effect is that C would be different with respect to the original value
> for A. Does a constraint that limits the allowable transitions for C imply
> constraints that limit the allowable transitions for A or B or both? Or
> should the transition be allowed because C was not targeted?
Received on Thu Sep 14 2006 - 18:34:21 CEST