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

From: paul c <toledobythesea_at_oohay.moc>
Date: Tue, 08 Mar 2005 04:11:27 GMT
Message-ID: <PB9Xd.600248$Xk.305685_at_pd7tw3no>

Dawn M. Wolthuis wrote:
> "paul c" <toledobythesea_at_oohay.moc> wrote in message
> news: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

>
>
> The first rule implies the second as you pointed out, but the second cannot
> stand in for the first as the implication goes only one direction (from the
> first rule to the second and not from the second statement of a rule to the
> first).
>
> --dawn
>
>
are you really saying that before the answer can get smaller, it has to get larger? (LOL)

p Received on Tue Mar 08 2005 - 05:11:27 CET