Re: RM formalism supporting partial information

"David BL" <davidbl_at_iinet.net.au> wrote in message news:649cc913-2425-4869-b803-0d2c59fe381d_at_a39g2000pre.googlegroups.com...
> On Nov 17, 7:54 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> > On 17 nov, 04:35, David BL <davi..._at_iinet.net.au> wrote:
>
> > > Also I think one could argue that the CWA is at odds with a model of
> > > partial information, unless you keep in mind the idea of "negation as
> > > failure to prove true".
> >
> > Here we touch the core of why I think your approach is problematic.
> > The CWA does apply for the does-not-apply interpretation of null
> > values. It does however not apply in its classical form for the value-
> > unknown interpretation. Actually Raymond Reiter himself (he introduced
> > the CWA) explains very well how it then changes in a paper with the
> > title "A sound and sometimes complete query evaluation algorithm for
> > relational databases with null values".
> >
> > What you seem to be doing is mixing the two interpretations, resulting
> > in something that IMO doesn't really seem to have any consistently
> > meaningful interpretation at all. If you really want to combine the
> > two interpretations you need to first define the meaning of a relation
> > with null values in terms of sets of "possible worlds" where the null
> > values are removed by either giving a concrete value for them or
> > declaring them not applicable. That might actually be quite
> > interesting, and I don't remember seeing a paper that did this
> > properly. Even Zaniolo doesn't really get this right, although he
> > claims that he combines the two approaches. So you are in very good
> > company. :-)
>
> The two main interpretations of null are apparently
>
> 1. value exists but is unknown
> 2. value doesn't exist
>

I would add my interpretation: value not stored here.

Where "here" is understood from the context of the NULL.

I'd go further. I'd say that both of the interpretations you listed above are inferences
based on my interpretation plus some presumed convention regarding the absence of a value.

As in "the dog did nothing in the night" (Arthur Conan Doyle).

The trouble arises when the some convention is presumed by the reader, but the writer intended some other convention or no convention at all.

> Zaniolo combines these into a single interpretation
>
> 3. no information
>

It sounds like my interpretation and Zaniolo's are the same. Received on Mon Nov 19 2007 - 06:31:02 CET