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 03:18:30 GMT
Message-ID: <asd2h.101$7F3.93_at_newssvr14.news.prodigy.com>
>> <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.
>>
>>
Date: Thu, 02 Nov 2006 03:18:30 GMT
Message-ID: <asd2h.101$7F3.93_at_newssvr14.news.prodigy.com>
"paul c" <toledobythesea_at_dbms.yuc> wrote in message news:pic2h.242958$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.
> p Received on Thu Nov 02 2006 - 04:18:30 CET