Re: two nasty schemata, union types and surrogate keys

From: Brian <brian_at_selzer-software.com>
Date: Tue, 22 Sep 2009 20:38:25 -0700 (PDT)
Message-ID: <945a2c53-58b4-4106-a416-b792bc846947_at_v2g2000vbb.googlegroups.com>



On Sep 22, 5:27 pm, r..._at_raampje.lan (Reinier Post) wrote:
> Brian wrote:
> >You're wrong, of course, but don't take my word for it.  According to
> >Date in /An Introduction to Database Systems, Eighth Edition/, page
> >161: 'the Closed World Assumption (also known as the Closed World
> >Interpretation) says that if an otherwise valid tuple--that is, one
> >that conforms to the relvar heading--does /not/ appear in the body of
> >the relvar, then we can assume the corresponding proposition is
> >false.
>
> But what *is* that proposition?  It might be
>
>   FIRTNAME LASTNAME is an employee at CORPORATION
>
> but it might just as well be
>
>   at some time in the past, it has been asserted that
>   FIRTNAME LASTNAME is an employee at CORPORATION
>
> which is a closed world formulation of what is approximately
> the open world counterpart of the first.

I don't think it is. Assuming that the assertion was true at that time in the past, the proposition is temporally qualified, whereas the first isn't.

>
> So just stating the closed world assumption isn't enough -
> you also have to rule out predicates that involve statements
> about assertions.

I really don't understand what you're trying to say: assertions are not terms, at least not in first-order logic.

>
> --
> Reinier
Received on Tue Sep 22 2009 - 22:38:25 CDT

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