What is a “data model”?

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Meaning of “data model” in the database world

From the book SQL and Relational Theory by C.J. Date:

The term “data model” has two quite distinct meanings in the database world. The first one is more fundamental.

The left-hand side of the following table explains the two different meanings. The right-hand side has an analogy that nicely illuminates the relationship between them.

A data model (first sense) is an abstract, self-contained, logical definition of the data structures, data operators, and so forth, that together make up the abstract machine with which users interact.	A data model in the first sense is like a programming language, whose constructs can be used to solve many specific problems but in and of themselves have no direct connection to any such specific problem.
A data model (second sense) is a model of the data—especially the persistent data—of some particular enterprise.	A data model in the second sense is like a specific program written in that language—it uses the facilities provided by the model, in the first sense of that term, to solve some specific problem.

If we’re talking about data models in the second sense, then we might reasonably speak of “relational models” in the plural. But if we’re talking about data models in the first sense, then there’s only one relational model, and it is the model.

Meaning of “data model” in the XML world

I think that a data model in the first sense is the set of constructs (complex types, simple types, element declarations, attribute declarations, etc.) defined in the XML Schema specification.

I think that a data model in the second sense is a particular XML Schema. For example, the Geography Markup Language (GML) schema is a data model in the second sense.

Do you agree?

/Roger