Re: Does Codd's view of a relational database differ from that ofDate&Darwin? [M.Gittens]

Alexandr Savinov wrote:
> Another point is that variable is something that stores a reference.

A variable stores a value. The value it stores can change over time. References and pointers are implementation "niceties" that are often overexposed; they are, in essence, variable-variables. Contrast with relational theory, in which relations are the only variables.

> This reference may point to
> - one object (record),
> - a collection of records which is dynamically defined (for example, a
> result of some query),
> - a table/domain which is statically defined in the schema
> - anything else that can be represented by reference

References without limitations, while useful in some meta-theory, are invitations to unrestrained graph theory. Not what most practicioners need, as relational and OO and other languages all reign this in in different ways.

> So you are right that domain is not a variable but reference to a domain
> can be stored in as many variables as we like just like we can store in
> variable references to records or result sets.

Your previous example showed a changing domain, n'est-ce pas?

> If we define null as absence then as a
> minor advantage we avoid problems with aggregation because absent things
> are simply not visible, i.e., null values are skipped.

So we can't count the number of "objects" that "contain" a "null value"?

> No, and this precisely what I wanted to emphasize. Because you cannot
> count what does not exist.

So an object can "contain" a null, but we can't be aware of that fact because the null cannot be counted? Is this some baroque application of Heisenberg's Uncertainty Theorem to data management? :-)

> The problem is to understand that different
> things may exist in different dimensions.

Projection is the usual reference here, but projections aren't completely unrestrained as you would have them.

> When we give such a definition then
> we get a very simple and effective data model (along with other principles).
>
> I am sorry, but the only thing I can advice is to read something about
> concept-oriented model. It is difficult because the description is bad
> (I actually lost an interest to it because almost everything is clear
> for me).

I wish you'd share it with the rest of us.

• erk