Re: two nasty schemata, union types and surrogate keys

From: Brian <brian_at_selzer-software.com>
Date: Mon, 21 Sep 2009 21:08:24 -0700 (PDT)
Message-ID: <3c415b4c-178b-44c0-8ef1-2a2b66380f98_at_o36g2000vbl.googlegroups.com>



On Sep 21, 4:19 pm, Roy Hann <specia..._at_processed.almost.meat> wrote:
> Brian wrote:
> > On Sep 21, 10:27 am, Roy Hann <specia..._at_processed.almost.meat> wrote:
> >> Brian wrote:
> >> > On Sep 21, 3:41 am, Roy Hann <specia..._at_processed.almost.meat> wrote:
> >> >> Brian wrote:
> >> >> > Databases don't record objects: they record facts [...].  
>
> >> >> Admirably close, but not quite cigar-worthy.  Databases record
> >> >> *assertions* of fact.  The assertsions may be sincere and truthful, or
> >> >> sincere but false, or deceitful and false.
>
> >> > Whatever.  What is in the database is supposed to be true.  
>
> >> Says who?  
>
> > Everyone who advocates the closed world assumption.
>
> The closed world assumption doesn't tell you anything about what is
> actually in the database; it tells you how you are entitled to
> manipulate what you find in the database.  I hinted at that in
> my first post when I wrote "All that matters is that we can make the
> inferences that we should be entitled to make from the assertions."

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. In other words, the body of the relvar at any given time contains /all/ and /only/ the tuples that correspond to true propositions at that time.' So the closed world assumption tells us that what is actually in the database is supposed to be true, while what is not is supposed to be false.

> >> It doesn't need to be true, and it can't be guaranteed to be
> >> true, so it is wise to remember that what's in the database is just
> >> claims and assertions.  It is completely sufficient that the database is
> >> consistent.
> > I don't think that it is sufficient.
>
> Well that's too bad, 'cos that's all you can have.
>
> --
> Roy- Hide quoted text -
>
> - Show quoted text -
Received on Mon Sep 21 2009 - 23:08:24 CDT

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