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

From: paul c <toledobythesea_at_oohay.moc>
Date: Wed, 09 Mar 2005 01:12:26 GMT
Message-ID: <_3sXd.615598$6l.498192@pd7tw2no>

Jan Hidders wrote:
> 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.
>
> -- Jan Hidders

Looks like the answer IS getting bigger, based on the word count.

p Received on Tue Mar 08 2005 - 19:12:26 CST