Re: Lucid statement of the MV vs RM position?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 02 May 2006 18:20:39 GMT
Message-ID: <XpN5g.2057$A26.59263_at_ursa-nb00s0.nbnet.nb.ca>
>> David Cressey wrote:
>>
>>> In Ted Codd's 1970 paper, he points out that when a system of
>>> relations is
>>> devised to store a body of facts, there are other systems of
>>> relations that
>>> will express precisely the same body of facts. He then points out that
>>> within a group of such systems that are all logically equivalent, there
>>> will be (at least) one that contains no sets, lists, or RVAs as
>>> elements of
>>> a tuple.
>>
>> ....on two conditions:
>>
>> "(1) The graph of interrelationships of the nonsimple
>> domains is a collection of trees.
>> (2) No primary key has a component domain which is
>> nonsimple.
>> The writer knows of no application which would require
>> any relaxation of these conditions."
>>
>> Other writers claim they *do* know of such applications, however, and
>> have proposed (semi-)formal guidelines for identifying cases where RVAs
>> may be appropriate.
>> ...
Date: Tue, 02 May 2006 18:20:39 GMT
Message-ID: <XpN5g.2057$A26.59263_at_ursa-nb00s0.nbnet.nb.ca>
> Jon Heggland wrote: >
>> David Cressey wrote:
>>
>>> In Ted Codd's 1970 paper, he points out that when a system of
>>> relations is
>>> devised to store a body of facts, there are other systems of
>>> relations that
>>> will express precisely the same body of facts. He then points out that
>>> within a group of such systems that are all logically equivalent, there
>>> will be (at least) one that contains no sets, lists, or RVAs as
>>> elements of
>>> a tuple.
>>
>> ....on two conditions:
>>
>> "(1) The graph of interrelationships of the nonsimple
>> domains is a collection of trees.
>> (2) No primary key has a component domain which is
>> nonsimple.
>> The writer knows of no application which would require
>> any relaxation of these conditions."
>>
>> Other writers claim they *do* know of such applications, however, and
>> have proposed (semi-)formal guidelines for identifying cases where RVAs
>> may be appropriate.
>> ...
>
> An example that some may not find very interesting, ie., too simple but
> still troubles me might be "the sets/combinations of parts that a
> supplier will agree to ship" having no other attributes than S# and P#,
> eg., SP { S#, {P#}} where I'm intending {P#} to mean a set of parts. I'm
> interested to know of the other writers or what they say.
While I can see no particular argument against SP { S#, {P#} } as a view, I can see some potential problems with having it as a base relation. Of course, those problems are only potential problems. Received on Tue May 02 2006 - 20:20:39 CEST