Re: need some help

From: paul c <toledobythesea_at_oohay.moc>
Date: Fri, 25 Feb 2005 21:35:13 GMT
Message-ID: <lSMTd.506443$8l.156274_at_pd7tw1no>


Jan Hidders wrote:
> 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

i started to think about this and didn't get much further than guessing that it is a question about covers. apologies for not answering the question literally, but i wonder how one would express an fd as a relation?

pc Received on Fri Feb 25 2005 - 22:35:13 CET

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