Re: "Armstrong's axioms" augmentation - help plz

From: paul c <toledobythesea_at_oohay.moc>
Date: Tue, 08 Mar 2005 03:41:07 GMT
Message-ID: <n99Xd.599930$Xk.252349_at_pd7tw3no>


Jan Hidders wrote:
> love boat via DBMonster.com wrote:
>

>> I understand the Augmentation rule:
>> { X -> Y } |= XZ -> YZ
>>
>> but I don't understand why the rule can also be stated as:
>>
>> { X -> Y } |= XZ -> Y
>>
>> Why is this?

>
>
> It cannot. If you replace the first rule with the second you will not
> derive all FDs that hold.
>
> -- Jan Hidders

The first 'rule' is X -> Y, and so is the second! What's the difference?

p Received on Tue Mar 08 2005 - 04:41:07 CET

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