Re: Basic question?What 's the key if there 's no FD(Functional Dependencies)?
From: Brian Selzer <brian_at_selzer-software.com>
Date: Thu, 02 Nov 2006 12:11:50 GMT
Message-ID: <agl2h.109$r12.54_at_newssvr12.news.prodigy.com>
> Alright, I'll risk displaying my ignorance and ask if such a relation has
> exactly one FD, is there a way to count to two? (and thus three, four ...)
>
Date: Thu, 02 Nov 2006 12:11:50 GMT
Message-ID: <agl2h.109$r12.54_at_newssvr12.news.prodigy.com>
"paul c" <toledobythesea_at_dbms.yuc> wrote in message
news:tOd2h.237313$5R2.133420_at_pd7urf3no...
> Brian Selzer wrote:
>> "paul c" <toledobythesea_at_dbms.yuc> wrote in message
>> news:pic2h.242958$R63.119230_at_pd7urf1no...
>>> Brian Selzer wrote:
>>>> <saturnlee_at_yahoo.com> wrote in message
>>>> news:1162158876.794350.29460_at_f16g2000cwb.googlegroups.com...
>>>>> I have a basic question.
>>>>> Suppose there are 3 attributes: A,B and C.
>>>>> And there are no FD(trivial and non trivial )
>>>>>
>>>>> What 's the key for it? ABC or nothing???
>>>>>
>>>> A relation with at least one attribute *ALWAYS* has at least one FD.
>>>> Here are some of them for your example:
>>>>
>>>> {ABC} --> A
>>>> {ABC} --> B
>>>> {ABC} --> C
>>>> {ABC} --> {ABC}
>>>>
>>>> Note that all of the FD's are trivial.
>>>>
>>>>
>>> Wouldn't a relation with no attributes also have a trivial FD?
>>>
>>
>> I'm not sure: I've never thought about it. Is {} --> {} trivial?
>> Probably so.
>
> Alright, I'll risk displaying my ignorance and ask if such a relation has
> exactly one FD, is there a way to count to two? (and thus three, four ...)
>
Since a functional dependency involves sets of attributes and since there is only one empty set, I would have to conclude that a relation with no attributes can have only one functional dependency, {} --> {}.
> p
Received on Thu Nov 02 2006 - 13:11:50 CET