paul c wrote:
> 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.
>>>
>>> The first 'rule' is X -> Y, and so is the second! What's the
>>> difference?
>>
>> 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).
>>
> are you really saying that before the answer can get smaller, it has to
> get larger? (LOL)
Actually what Dawn was telling you is that if you want to get a bigger
answer you need a rule that makes the answer bigger. Makes sense, no?
The first rule allows you to do that, the second doesn't. Knowing that
it's pretty simple to come up with a formal proof that you can derive
less dependencies if you replace the first rule with the second one.
Received on Tue Mar 08 2005 - 14:51:41 CST
