Re: ID field as logical address

From: Brian Selzer <>
Date: Mon, 1 Jun 2009 13:05:48 -0400
Message-ID: <MTTUl.27897$>

"Bernard Peek" <> wrote in message
> In message <ImwUl.30133$PH1.14509_at_edtnps82>, paul c
> <> writes

>>Walter Mitty wrote:
>>> "Brian Selzer" <> wrote in message
>>>> {{L:Smith,  F:Mary, Stat:Single},
>>>> {L:Jones, F:Mary, Stat:Married}}
>>>> and is then assigned the value,
>>>> {{L:Smith, F:Mary, Stat:Divorced},
>>>> {L:Jones, F:Mary, Stat:Married}}
>>>> ...
>>>> Note that the introduction of an autogenerated ID eliminates all 
>>>> confusion:
>>>> ...
>>"autogenerated ID eliminates all confusion", what a laugh, almost sounds 
>>like a sleeping pill slogan.

> I think I'd say that an autogenerated ID hides the problem, or at least
> moves it around. Instead of asserting that the key always refers to an
> instance within the universe of discourse, you substitute an autonumber
> field and make the same assertion.

Not so: addition is not the same as substitution.


> In an entity the key refers unambiguously to an instance within the
> universe. When you build a database table that is an aspiration.

Consider an ordered set, such as a deck of cards. When implemented in a relation schema, {Position, Card}, there are two candidate keys, {Position}, and {Card}. Now suppose that the deck is shuffled, and a database update issued to reflect the new state of the deck. The card at the bottom of the deck before shuffling may not be the card at the bottom of the deck after shuffling. So despite the fact that Position is a candidate key of the representative relation schema, instances of it do not permanently identify things in the Universe.

> --
> Bernard Peek
Received on Mon Jun 01 2009 - 19:05:48 CEST

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