Re: Separate PK in Jxn Tbl?
Date: Mon, 28 Jan 2008 23:20:20 GMT
Message-ID: <Uqtnj.1878$0w.1841_at_newssvr27.news.prodigy.net>
"Roy Hann" <specially_at_processed.almost.meat> wrote in message
news:9LmdncQYbI5rogPaRVnyvwA_at_pipex.net...
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:odonj.910$R84.571_at_newssvr25.news.prodigy.net...
>>
>> I don't think it's productive to blame it on the users. The problem
>> you're referring to is due to the nature of keys, and misunderstandings
>> on the part of the database designer as to what constitutes a key. All
>> that is required for a key to be a key is that in every possible database
>> instance, a projection over the attributes in the key for a relation has
>> the same cardinality as the relation. This does /NOT/ mean that a
>> particular combination of values /always/ identifies the same individual
>> in the Universe of Discourse, but only in the picture of the Universe
>> that is a database instance. In other words, a particular combination of
>> values may not /necessarily/ identify an individual, but rather may only
>> /contingently/ identify an individual. 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.
>
> Roy
>
Received on Tue Jan 29 2008 - 00:20:20 CET