Re: A pk is *both* a physical and a logical object.
Date: Sat, 14 Jul 2007 10:55:25 -0300
David Cressey wrote:
> "Jan Hidders" <hidders_at_gmail.com> wrote in message > news:1184241371.515071.251680_at_k79g2000hse.googlegroups.com... >
>>On 11 jul, 22:25, Cimode <cim..._at_hotmail.com> wrote:
>>><<Technically a PK is *only* a physical implementation device, not a
>>>logical concept at all.>>
>>`When I use a word,' Humpty Dumpty said, in rather a scornful tone,
>>`it means just what I choose it to mean -- neither more nor less.'
>>`The question is,' said Alice, `whether you can make words mean so
>>many different things.'
>>`The question is,' said Humpty Dumpty, `which is to be master --
>>To answer the question, I think that is quite simple. As defined in
>>the relational model it is a logical concept. As far as I know the SQL
>>standard does not state that a PK implies an index (but I could be
>>wrong) and then it is also there a logical concept. If it does imply
>>an index then it is mixed concept because it has both logical and
>>-- Jan hidders
> > It was my understanding that the relational model defines keys, but not > primary keys. That is, any candidate key is as much of a key as any other.
I believe primary keys were introduced and accepted by relational theorists, and the concept had a lot of traction for quite a long time. In the end, they had no theoretical underpinning apart from the theory underpinning the concept of candidate key, and their perceived importance has declined. Received on Sat Jul 14 2007 - 15:55:25 CEST