SQL and Relational Theory, 2nd Edition

Preface to the First Edition

SQL is ubiquitous. But SQL is hard to use: It’s complicated, confusing, and error prone (much more so, I venture to suggest, than its apologists would have you believe). In order to have any hope of writing SQL code that you can be sure is accurate, therefore—meaning it does exactly what it’s supposed to do, no more and no less—you must follow some appropriate discipline. And it’s the thesis of this book that using SQL relationally is the discipline you need. But what does this mean? Isn’t SQL relational anyway?

Well, it’s true that SQL is the standard language for use with relational databases—but that fact in itself doesn’t make it relational. The sad truth is, SQL departs from relational theory in all too many ways; duplicate rows and nulls are two obvious examples, but they’re not the only ones. As a consequence, the language gives you rope to hang yourself with, as it were. So if you don’t want to hang yourself, you need to understand relational theory (what it is and why); you need to know about SQL’s departures from that theory; and you need to know how to avoid the problems they can cause. In a word, you need to use SQL relationally. Then you can behave as if SQL truly were relational, and you can enjoy the benefits of working with what is, in effect, a truly relational system.

Now, a book like this wouldn’t be needed if everyone was using SQL relationally already—but they aren’t. On the contrary, I observe much bad practice in current SQL usage. I even observe such practice being recommended, in textbooks and similar publications, by writers who really ought to know better (no names, no pack drill); in fact, a review of the literature in this regard is a pretty dispiriting exercise. The relational model first saw the light of day in 1969, and yet here we are, over 40 years later, and it still doesn’t seem to be very well understood by the database community at large. Partly for such reasons, this book uses the relational model itself as an organizing principle; it explains various features of the model in depth, and shows in every case how best to use SQL in order to comply with the feature in question.