Re: object algebra

"Neo" <neo55592_at_hotmail.com> wrote in message news:4b45d3ad.0402231436.5674aabe_at_posting.google.com...
> > > RDM isn't fully general. If it is, why does it need NULLs?
> >
> > It is fully general, and it doesn't need nulls.
>
> According to Date "Codd now regards NULLs as an integral part of
> relational model". Codd is correct that NULLs are required in RDM,
> unless all your data fits neatly in rectangular tables which is not
> the general case, or unless one resorts to unpractical methods such as
> generic modelling (ie all data in one table with one column).

Do you make some distinction between relational and RDM, and if so, what is RDM? Tables are not relations, and neither of them are "rectangular." You can display, for example, a 4-dimensional tessaract (hypercube) as a table if you like - that doesn't mean it's rectangular.

Generic modeling is a contradiction in terms - all data in one table with one column models nothing, unless you're discussing meta-modeling of some sort.

The general case is for data to fit into relations - if you can't do that, you're being exceptionally sloppy with what you're saying (and failing to say) about your company's data. You can be sufficiently general through the attribute types (and subtypes!) that your relations are defined over.

• Eric