Re: Nulls, integrity, the closed world assumption and events

From: JOG <jog_at_cs.nott.ac.uk>
Date: 12 Jan 2007 05:15:39 -0800
Message-ID: <1168607737.058370.24420@q2g2000cwa.googlegroups.com>

David wrote:

> Brian Tkatch wrote:
> > David wrote:
> > > Brian Tkatch wrote:
> > > > Cimode wrote:
> > > > > David wrote:
> > > > > > Consider the following relation
> > > > > >
> > > > > > person(P,M,F) :- person P has mother M, father F.
> > > > > >
> > > > > > By induction a non-empty
> > > > > > database would have to be infinite.
> > > > > A false premise makes all deductions coming from it false. Closed
> > > > > World Assumption does not say anything about the number of element that
> > > > > belong to a domain of values from which one attribute values are
> > > > > derived. In the case of a *person* domain, the number of elements in
> > > > > the set is certainly finite. Therefore, the number of propositions
> > > > > involving person as an attribute is limited as well.
> > > >
> > > > Would not the question be better stated:
> > > >
> > > > If a table lists linked chains (with the linked-to link), how is the
> > > > final link (whether first or last) stated?
> > > >
> > > > In such a case i link the item to itself.
> > >
> > > You suggest a proposition stating that a person is their own
> > > mother/father?
> >
> > Yes.
> >
> > In a cause-effect table that records all causes and effects has to
> > assume a circular relationship for either the prime cause or (current)
> > final effect.
> >
> > If the table in question here was to denote parents and their children,
> > there would be allowance for an entry of no children, thus ending the
> > chain. However, the case here is people and their parents, without the
> > option of no parent.
> >
> > This means the chain must be circular, the question is to which link.
> > Being it would create an impossible relationship for a parent to have a
> > child as his parent, the circular relationship must be to itself.
> >
> > So yes, i would indeed suggest a proposition stating that a person is
> > their own parent.

>

> Do you suggest this by analogy to algorithms and data structures
> (written in C++ for example) that use the same technique?
> Interestingly the more common approach is to use null pointers.

>

> IMO the RM is founded first and foremost on mathematical logic, and
> therefore stating any proposition that is actually wrong must be
> avoided. This is also why I think nulls are bad.

>

> Whilst on the subject of nulls, I have seen a paper written in '83
> by Carlo Zaniolo that suggests nulls can represent "no information"
> (which encompasses all the more specific interpretations such as
> "unknown" and "non-existent"), and this appears to lead to a
> decent mathematical model, unlike Codd's 3vl which is far from
> compelling.

Interesting. However I'd contest that there can any concept of 'missing information' at all at the logical level (where either we know a fact or we don't). It seems to me that information may only be deemed 'missing' at the conceptual level, and that for the database layer proper the term is a misnomer that causes a lot of confusion. Received on Fri Jan 12 2007 - 07:15:39 CST