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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@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 Mon Mar 07 2005 - 21:41:07 CST












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