Re: Separate PK in Jxn Tbl?
Date: Tue, 29 Jan 2008 09:38:47 -0000
"Brian Selzer" <brian_at_selzer-software.com> wrote in message
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
>>>> No, the position number identifies the same position, for all time.
>>>> The facts about the current occupant of the position may change
>>> 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.
No. The number on your driver's license identifies only the license. The (more-or-less) reliable mapping between the license and you is not a result solely of the uniqueness of the licence number. If that were sufficient you could just walk into a registration office and tear off the next unique number from a roll and walk out. But there is a whole business process that (is presumed) to map your identity to the licence number. (Here we could shoot off into the long grass of identity fraud and biometrics--but let's not.)
> 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?
Only in the colloquial sense, as when buying liquor or being pulled over for DUI. If someone believes that they can use the photo on the license document to match you to the other data on the license document good luck to them. But they are not using the license number to identify you.
Roy Received on Tue Jan 29 2008 - 10:38:47 CET