Re: RM formalism supporting partial information

On Nov 27, 9:43 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> On 26 nov, 15:06, David BL <davi..._at_iinet.net.au> wrote:
> > On Nov 26, 7:47 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> > > On 26 nov, 08:52, David BL <davi..._at_iinet.net.au> wrote:
>
> > > > Firstly a minor nit pick: you can't say "possible answers", because
> > > > they don't actually represent an upper bound on the result in the
> > > > omniscient database.
>
> > > ?? They do so by definition.
>
> > What I meant was that unless CWA is available on an appropriate
> > projection there may be so much missing information (eg all
> > information about an entity) that the query purported to return the
> > "possible answers" does no such thing. ie it suffers a similar
> > problem to negation (it returns neither the certain nor the possible
> > answers).
>
> I'm not sure what you mean by "the query purported to return the
> 'possible answers'". If the user formulates a query then this will now
> include an indication of whether he or she wants the possible/certain
> answers. It is up to the DBMS to efficiently compute the answer, and
> this is not necessarily done by the usual translation of calculus to
> algebra or even one very similar to it.

Consider a query to find all the 27 year old pilots from a census recorded in an RDB. If the age or occupation is missing we could think of the person as a possible answer. However we cannot say the query returns all possible answers unless we assume every person took part in the census. Received on Wed Nov 28 2007 - 01:58:01 CET