Re: Does Codd's view of a relational database differ from that ofDate&Darwin? [M.Gittens]
Date: 7 Jun 2005 09:02:26 -0700
Message-ID: <1118160146.522532.196570_at_g47g2000cwa.googlegroups.com>
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
> 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?
> 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.
> The problem is to understand that different
> things may exist in different dimensions.
> 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