Re: A pk is *both* a physical and a logical object.
From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 28 Jul 2007 03:10:54 GMT
Message-ID: <2tyqi.11337$rX4.9610_at_pd7urf2no>
>>David Cressey wrote:
>>
>>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>>news:DjPpi.24618$Rw1.11254_at_newssvr25.news.prodigy.net...
>>>
>>>
>>>
>>>>This illustrates what happens when the only key on a relation schema
>>>
>>>permits
>>>
>>>
>>>>updates. It can't be determined if a new individual is being selected,
>>>>or
>>>>if the state of the current individual is now different.
>>>
>>>
>>>This is the point I have been trying to make for the past week or so.
>>>The
>>>mathematics of the relational data model don't, in this case,
>>>disambiguate
>>>two profoundly different scenarios in the real world the data purports to
>>>describe.
>>>...
>>
>>David, what does it matter?
>>
>>The user/audience can agree to disambiguate/interpret however it suits
>>their purpose/application.
>>
>>What is the possible usefulness of using this term "rigid" to describe a
>>key?
>>
Date: Sat, 28 Jul 2007 03:10:54 GMT
Message-ID: <2tyqi.11337$rX4.9610_at_pd7urf2no>
Brian Selzer wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message > news:wo1qi.7977$fJ5.772_at_pd7urf1no... >
>>David Cressey wrote:
>>
>>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>>news:DjPpi.24618$Rw1.11254_at_newssvr25.news.prodigy.net...
>>>
>>>
>>>
>>>>This illustrates what happens when the only key on a relation schema
>>>
>>>permits
>>>
>>>
>>>>updates. It can't be determined if a new individual is being selected,
>>>>or
>>>>if the state of the current individual is now different.
>>>
>>>
>>>This is the point I have been trying to make for the past week or so.
>>>The
>>>mathematics of the relational data model don't, in this case,
>>>disambiguate
>>>two profoundly different scenarios in the real world the data purports to
>>>describe.
>>>...
>>
>>David, what does it matter?
>>
>>The user/audience can agree to disambiguate/interpret however it suits
>>their purpose/application.
>>
>>What is the possible usefulness of using this term "rigid" to describe a
>>key?
>>
> > > It is a simple and precise term that describes a class of identifiers. > There are keys whose values identify a specific individual at all database > values, and there are keys whose values identify a specific indivdual at > some database values. For example, a relation that models an ordered set > has two keys, one that represents names for elements and one that represents > positions for elements. Both meet all of the criteria for a candidate key > (uniqueness and irreducibility), but only the one that represents names > permanently identifies each element, since at different database values, a > particular element may be in different positions. >
I repeat, what does it matter? If it happens to be a simple and precise term, so what, eg., what is the point? What is the possible use? Eg., why would this notion ever matter to a dbms?
If somebody will please answer this question, I'll stop asking it!
p Received on Sat Jul 28 2007 - 05:10:54 CEST