Re: Interpretation of Relations

paul c wrote:
> JOG wrote:
> > Joe Thurbon wrote:
> >
> >>I'm very new to this databases game, and am not even sure I'm using the
> >>terminology in the right way. I'd like some feedback as to whether I'm
> >>even in the right ballpark. Most of my understanding of the terminology
> >>comes from reading this group, and the definitions on Wikipedia.
> >>
> >>I've been wrestling with the correct interpretation of a relation. I'm
> >>currently working under the assumption that a relation comprises both a
> >>Type (or header) and a Body (or values).
> >>
> >>I'm going to start small.
> >>
> >>Consider modelling a situation in which there are people, and they have
> >>eye colour. I'm going to define some very small domains, so that I can
> >>enumerate the facts that I believe a given relation represents. I'm
> >>sorry if the notation is non-standard, but here it is.
> >>
> >>Domain D_People = {Joe}
> >>Domain D_Hair = {Red, Blond}
> >>
> >>Relation R_People = <<D_People>: {{Joe}}>
> >>Relation R_Hair Colour = <<D_People X D_Hair>: {{Joe, Blond}}>
> >>
> >>(The bit in the <> is the relation header, the subsequent sets are the
> >>relation body).
> >>
> >>So, I should interpret this to mean that "Joes hair is blond".
> >>But that is not all, because the closed world assumption means that I
> >>have another fact (just one, because the domains are so small). This
> >>second fact is "NOT Joes hair is red".
> >>
> >>And, in particular, it would be wrong to state that
> >>
> >>R_Hair Colour: <<D_People X D_Hair>: {}>
> >>
> >>indicates that I don't know the colour of Joe's hair. It really means
> >>
> >>NOT Joes hair is Red
> >>NOT Joes hair is Blond
> >>
> >>Is this right?
> >
> >
> > I see nothing wrong with your logic. What you are saying via an empty
> > relation is that there is no proposition for which Joe has a hair
> > colour. In fact I think from it you can infer: Joe has no hair colour.
> > (which given my external knowledge --> Joe has no hair).
> >
> >
> >>If so, it leads me to a question about modelling missing
> >>information. (And a lot of other questions, too). If not, is there a
> >>simple thing that I've missed?
> >
> >
> > My current understanding is that:
> >
> > * If an attribute is Inapplicable then simply stating no proposition
> > containing it is sufficient for that fact to be inferred.
> > * If the attribute is Applicable but we do not have value for it, then
> > we must state this propositionally to avoid inferring it as
> > inapplicable under the CWA.
> > * Alternatively if the attribute is 'possible' (we don't know if it is
> > missing or inapplicable) then we must also state a proposition
> > reflecting this in order to avoid the inapplicability inference.
> >
> > It is worth noting that many view db-query results as coming with the
> > caveat "as far as I, poor naive database, know". Jim.
> >
> > ...

>

charging for what? His haircut? Received on Sat Jan 20 2007 - 03:47:23 CET