Re: Separate PK in Jxn Tbl?
Date: Tue, 29 Jan 2008 08:20:07 GMT
Message-ID: <XkBnj.836$0o7.298_at_newssvr13.news.prodigy.net>
"Roy Hann" <specially_at_processed.almost.meat> wrote in message
news:dbSdnZp48tMySgPanZ2dnUVZ8tOmnZ2d_at_pipex.net...
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:Uqtnj.1878$0w.1841_at_newssvr27.news.prodigy.net...
>>
>> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
>> news:9LmdncQYbI5rogPaRVnyvwA_at_pipex.net...
>>> This is the nature of keys:
>>>> either the values for a key are permanent identifiers, or they're not.
>>>> It has nothing to do with how well keys are managed. The values for a
>>>> key may be managed perfectly, yet still not be permanent
>>>> identifiers--the position of something in a list of things comes to
>>>> mind.
>>>
>>> No, the position number identifies the same position, for all time. The
>>> facts about the current occupant of the position may change willy-nilly.
>>
>> I don't think so. In the domain of positions, a position number
>> identifies the same position, for all time: that is the nature of a
>> domain. But whenever a position number appears as a key in a relation,
>> it identifies an occupant, even though the occupant identified may not be
>> the same occupant at different database instances: this is the nature of
>> a key.
> > A key as you defined it earlier is sufficient to provide only > addressability, not identification. They are not the same thing > (although anything that provides identification would also be a key). > This is a basic part of the concept of functional dependency. >
What do you mean? What I have for identification is a driver's license. It has a driver's license number on it, which singles me out as a driver in my State. Several years ago, I had a different driver's license with a different driver's license number on it, but that one expired. The one I have now will also expire. Obviously, a driver's license number isn't a permanent identifier: are you then saying that a driver's license isn't sufficient for identification? I also have a voter's identification card, which has a number that singles me out as a voter in my State, locality and precinct. I had a different one a couple years ago, and I'll get a different one in a couple years. Are you saying that my voter's identification card isn't sufficient for identification? According to the Board of Elections, all I have to do is present it in order to vote. If I didn't have one, I would have to produce a driver's license or State identification along with a utility bill.
A functional dependency is a statement that says that whenever two tuples from the same relation agree on values for one set of attributes, they must also agree on values for another set of attributes. Note that it doesn't say that whenever two tuples agree, but rather that whenever two tuples /from the same relation/ agree.
> Roy
>
Received on Tue Jan 29 2008 - 09:20:07 CET