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

mountain man wrote:
> ======[quoting M.Gittens]=====================
>
> According to E.F. Codd a relational database is defined as:
>
> "A relational database is a time-varying collection of data,
> all of which can be accessed and updated as if they were
> organized as a collection of tabular time-varying tabular
> (nonhierarchic) relations of assorted degrees defined on a
> given set of simple domains."
>
> According to the Third manifesto:
>
> "The question as to what data types are supported is orthogonal
> to the question of support of the relational model."
>
> Let us take time to notice the fundamental difference between the
> two positions presented here. As a result of the position that
> support for the relational model is orthogonal to the supported
> data types, The Third Manifesto proceeded to allow domains to have
> an arbitrarily complex structure and also to support arbitrarily
> complex user defined operators.

True enough, but the terms "complex" and "simple" are highly subjective. Yes, you can define domains that are complex, but they're "simple" in this way: they're opaque to the RDBMS. They're black boxes. And I don't think you can prescribe or proscribe enough to keep people from designing badly.

> Codd on the other hand specifically states that relational
> databases must be based on "simple" domains.

Does he define "simple" anywhere? If not, that's a problem - we don't know exactly what he meant.

> Codd also says that
> domain values should not be decomposable further by the DBMS.

Correct, and Date says the same.

> Which
> is to say: According to Codd, the question as to what data types
> are supported is not orthogonal to the question of support of the
> relational model.

You're basing that conclusion on the word "simple," which if not defined is fairly useless. Which paper did Codd's definition come from? I know that Codd has allowed relation-valued attributes (RVAs) in other papers, but don't know the chronology and evolution of his definitions.

> Based on this evidence one can but conclude that
> Codd's view of relational database is logically different from The
> Third Manifesto's view of a relational database. Such logical
> differences are big differences.

Agreed, if they exist, but the above isn't convincing. While Date and Codd definitely disagree on nulls, it's hard to say much about the conclusions Gittens draws from the above.

• Eric