Note: The bulk of this section consists of an abbreviated and slightly revised version of material from an interview I did in 2005 (published in my book Date on Database: Writings 2000-2006, Apress, 2006).
The relational model, whatever else it might be, is certainly a theory—so I’d like to say a few words about the significance of theory in general before getting into details of the relational model in particular. As I said in the preface to the present book, it’s an article of faith with me that theory is practical. The purpose of relational theory in particular is not just theory for its own sake; the purpose of that theory is to allow us to build systems that are 100 percent practical. Thus, I believe that, in the relational context specifically, departures from the underlying theory are A Big Mistake.
Unfortunately, however, the term “theory” has two quite different meanings. In common parlance, it’s almost pejorative—“oh, that’s just your theory.” Indeed, in such contexts it’s effectively just a synonym for opinion (and the adverb merely—it’s merely your opinion—is often implied, too). But to a scientist, the term has a very different meaning. To a scientist, a theory is a set of ideas or principles that explain some set of observable phenomena, such as the motion of the planets. Of course, when I say I it explains something, I mean it does so coherently: It fits the facts, as it were. Moreover (and very importantly), it doesn’t just explain something, it also makes predictions—predictions that can be tested and (at least in principle) can be shown to be false. And if any of those predictions do indeed turn out to be false, then we move on: Either we modify the existing theory, or we adopt a new one. That’s the scientific method:
First, we observe certain phenomena, empirically.
We construct a theory or hypothesis to explain those phenomena.
We use that theory to make predictions.
We test the accuracy of those predictions.
Based on the results of those tests, we refine our theory (or reject it, in extreme cases).
And we iterate.
That’s how the Copernican system replaced epicycles; how Einstein’s cosmology replaced Newton’s; how general relativity replaced special relativity; and so on. Incidentally, Carl Sagan has a nice observation in this regard:
In science it often happens that scientists say, “You know, that’s a really good argument, my position is mistaken,” and then they actually change their minds, and you never hear that old view from them again. They really do it. It doesn’t happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.
Anyway, I claim the relational model is indeed a theory in the scientific sense; more specifically, I claim it’s a mathematical theory. Now, mathematical theories are a little special, in a way. First of all, the observed phenomena they’re supposed to explain tend to be rather abstract—not nearly as concrete as something like the motion of the planets, for example. Second, the predictions they make are essentially the theorems that can be proved within the theory; thus, those “predictions” can be falsified only if there’s something wrong with the premises, or axioms, on which the theorems are based. But even this does happen from time to time! For example, in euclidean geometry, you can prove that every triangle has angles that sum to 180 degrees. So if we ever found a triangle that didn’t have this property, we would have to conclude that the premises—the axioms of euclidean geometry—must be wrong. And in a sense exactly that happened: Triangles on the surface of a sphere (for example, on the surface of the Earth) turned out to have angles that sum to more than 180 degrees. And the problem turned out to be the euclidean axiom regarding parallel lines. Riemann replaced that axiom by a different one and thereby defined a different (but equally valid) kind of geometry.
In the same kind of way, the theory that’s the relational model might be falsified in some way—but I think it’s pretty unlikely, because (as I said in the previous section) the premises on which the relational model is based are essentially those of set theory and predicate logic, and those premises have stood up pretty well for a very long time.
So, to get to the real point of this section: Given that the relational model is a scientific theory, the question is whether that theory is really important. Of course, my own answer to this question is yes. In fact, I’d like to turn the question on its head ... First of all, database management is a field in which some solid theory does exist. Furthermore, we know the value of that theory; we know the benefits that accrue if we follow that theory. We also know there are costs associated with not following that theory (we might not know exactly what those costs are—I mean, it might be hard to quantify them—but we do know there are going to be costs).
If you’re traveling on an airplane, you’d like to be sure it’s been constructed in accordance with the principles of physics and aerodynamics. If you live or work in a high rise building, you’d like to be sure it’s been constructed in accordance with sound engineering and architectural principles. In the same kind of way, if you’re using a DBMS, wouldn’t you like to be sure it’s been constructed in accordance with solid database principles? If it hasn’t, you know things will go wrong. And while it might be hard to say exactly what will go wrong, and it might be hard to say whether things will go wrong in a major or minor way, you know—it’s guaranteed—that things will go wrong.
So I don’t think people should be asking “What’s the business value of implementing the relational model?” Rather, I think they should be asking, or perhaps trying to explain, what the business value is of not implementing it. In other words, those who ask “What’s the value of the relational model?” are basically saying “What’s the value of theory?”—and I hereby challenge them to tell me what the value is of not abiding by the theory.