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_at_pd7tw2no>
>
>
> 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
Date: Wed, 09 Mar 2005 01:12:26 GMT
Message-ID: <_3sXd.615598$6l.498192_at_pd7tw2no>
Jan Hidders wrote:
> paul c wrote:
>
>> Dawn M. Wolthuis wrote: >> >>> "paul c" <toledobythesea_at_oohay.moc> wrote in message >>> news:n99Xd.599930$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 Wed Mar 09 2005 - 02:12:26 CET