SQL and Relational Theory 2010

SQL and Relational Theory: How to write accurate SQL Code

Chris Date is the world's best known relational advocate. In this new seminar, he shows you how to write SQL code that's logically correct; how to avoid various SQL traps and pitfalls; and, more generally, how to use SQL as if it were a true relational language.

SQL is ubiquitous. But SQL is complicated, difficult, and error prone (much more so than SQL advocates would have you believe), and testing can never be exhaustive. So to have any hope of writing correct SQL, you must follow some discipline. What discipline? Answer: The discipline of using SQL relationally. But what does this mean? Isn't SQL relational anyway?

Well, of course SQL is the standard language for use with relational databases - but that doesn't make it relational! The sad truth is, SQL departs from relational theory in all too many ways; duplicate rows and nulls provide two obvious examples, but they're not the only ones. Thus, systems based on SQL give you rope to hang yourself, 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.

Of course, a seminar like this wouldn't be needed if everyone already used SQL relationally - but they don't. On the contrary, there's a huge amount of bad practice to be observed in current SQL usage. Such practice is even recommended in textbooks and other publications, by writers who really ought to know better; 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 - yet here we are, almost 40 years later, and it sti