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

From: Dawn M. Wolthuis <dwolt_at_tincat-group.comREMOVE>
Date: Mon, 7 Mar 2005 22:08:16 -0600
Message-ID: <d0j8fm$omh$1@news.netins.net>

"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 Received on Mon Mar 07 2005 - 22:08:16 CST