Re: need some help

jimi_xyz_at_hotmail.com wrote:
> Hi, I need some help with this database question..
>
> suppose s(R) = {A, B, C, D}
> and suppose FDs
> A,B->C; C->D; D->A
>
> A)What are the keys?
> B)What are the superkeys that are not keys?

Here's a hint:

    If (1) X is a superkey and (2) A->B holds and (3) B is in X     then X minus B plus A is also a superkey.

For example, if (1) BCD is a superkey and (2) AB->C holds and (3) C is in BCD then BCD - C + AB = ABD is also a superkey.

With that rule you can find all the superkeys by starting with the trivial superkey ABCD and applying this rule. If you find new superkeys then you apply the same procedure to them and repeat until you find no more new superkeys.

Once you have all the superkeys, you can easily determine the (candidate) keys because those are the ones for which there is no superkey which is a proper subset.

Good luck,

• Jan Hidders