SQL and Relational Theory, 2nd Edition

MODEL vs. IMPLEMENTATION

Before going any further, there’s an important point I need to explain, because it underpins everything else to be discussed in this book. The relational model is, of course, a data model. Unfortunately, however, this latter term has two quite distinct meanings in the database world. The first and more fundamental one is this:

Definition: 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.

This is the meaning we have in mind when we talk about the relational model in particular. And, armed with this definition, we can usefully, and importantly, go on to distinguish a data model in this first sense from its implementation, which can be defined as follows:

Definition: An implementation of a given data model is a physical realization on a real machine of the components of the abstract machine that together constitute that model.

Let me illustrate these definitions in terms of the relational model specifically. First of all, consider the concept relation itself. That concept is part of the model: Users have to know what relations are, they have to know they’re made up of tuples and attributes, they have to know how to interpret them, and so on. All that’s part of the model. But they don’t have to know how relations are physically stored on the disk, or how individual data values are physically encoded, or what indexes ...