SQL and Relational Theory, 2nd Edition

THE RELATIONAL MODEL DEFINED

Now I’d like to give a precise definition of just what it is that constitutes the relational model. The trouble is, the definition I’ll give is indeed reasonably precise: so much so, in fact, that I think it would have been pretty hard to understand if I’d given it in Chapter 1. (As Bertrand Russell once memorably said: Writing can be either readable or precise, but not at the same time.) Now, I did give a definition in Chapter 1—a definition, that is, of what I there called “the original model”—but I frankly don’t think that definition is even close to being good enough, for the following reasons among others:

• For starters, it was much too long and rambling. (Well, that was fair enough, given the intent of that preliminary chapter; but now I want a definition that’s reasonably succinct, as well as being precise.)

• I don’t really much care for the idea that the model should be thought of as consisting of “structure plus integrity plus manipulation”; in some ways, in fact, I think it’s actively misleading to think of it in such terms. The truth is, those three aspects of the model are inextricably intertwined. For example, the relvars in any given database (structural piece) will be subject to a variety of integrity constraints (integrity piece), and those constraints will be expressed using a variety of relational operators (manipulative piece). Moreover, those relvars will (or should) have been designed in accordance with relational design theory, which ...