Re: two nasty schemata, union types and surrogate keys
Date: Mon, 19 Oct 2009 19:19:24 -0700 (PDT)
Message-ID: <e722106f-84b7-41e0-b33a-cb6db1fffe53_at_m33g2000pri.googlegroups.com>
On Oct 20, 7:30 am, Brian <br..._at_selzer-software.com> wrote:
>
> Under the closed world intepretation, every formula that can be
> represented in a table is assigned a truth value--positive for those
> that are actually represented in the table and negative for those that
> aren't, but under the open world interpretation, only those that are
> actually represented are assigned truth values. Let's put it another
> way: either it is supposed to be true or it is known to be true.
> Under the closed world interpretation, what is represented is supposed
> to be true, but under the open world interpretation, what is
> represented is known to be true. Bottom line: it would be pointless
> to suppose that what is represented is known to be true.
Consider two external predicates p1, p2 (where "external" means they are informally described in natural language) satisfying
for all X, p1(X) --> p2(X)
E.g.
p1(X) :- X is a frog currently on display in
the San Diego zoo
p2(X) :- X is an amphibian currently on display
in the San Diego zoo
A relvar recording p1 under the CWA can also be regarded as recording p2 under the OWA.
Putting it another way, it is often the case that by "narrowing" the
external predicate one can turn an OWA into a CWA.
E.g. there may be a relvar for which the following gives an OWA:
p2(X) :- X is currently an employee of Acme Co
whereas the following gives a CWA:
p1(X) :- It is known to the HR department that X is currently
an employee of Acme Co.