SQL and Relational Theory, 2nd Edition

THE RELATIONAL MODEL IS MUCH MISUNDERSTOOD

Professionals in any discipline need to know the foundations of their field. So if you’re a database professional, you need to know the relational model, because the relational model is the foundation (or a large part of the foundation, at any rate) of the database field in particular. Now, every course in database management, be it academic or commercial, does at least pay lip service to the idea of teaching the relational model—but most of that teaching seems to be done very badly, if results are anything to go by. Certainly the model isn’t well understood in the database community at large. Here are some possible reasons for this state of affairs:

• The model is taught in a vacuum. That is, for beginners at least, it’s hard to see the relevance of the material, or it’s hard to understand the problems it’s meant to solve, or both.

• The instructors themselves don’t fully understand or appreciate the significance of the material.

• Perhaps most likely in practice, the model as such isn’t taught at all—the SQL language, or some specific dialect of that language, such as the Oracle dialect, is taught instead.

So this book is aimed at database practitioners in general, and SQL practitioners in particular, who have had some exposure to the relational model but don’t know as much about it as they ought to, or would like to. It’s definitely not meant for beginners; however, it isn’t just a refresher course, either. To be more specific, I’m sure you know something about SQL; but—and I apologize for the possibly offensive tone here—if your knowledge of the relational model derives only from your knowledge of SQL, then I’m afraid you won’t know the relational model as well as you should, and you’ll probably know “some things that ain’t so.” I can’t say it too strongly: SQL and the relational model aren’t the same thing. Here by way of illustration are some relational issues that SQL isn’t too clear on (to put it mildly):

• What databases, relations, and tuples really are

• The difference between relation values and relation variables

• The relevance of predicates and propositions

• The importance of attribute names

• The crucial role of integrity constraints

• The Information Principle and its significance

and so on (this isn’t an exhaustive list). All of these issues, and many others, are addressed in this book.

I say again: If your knowledge of the relational model derives only from your knowledge of SQL, then you might know “some things that ain’t so.” One consequence is that you might find, in reading this book, that you have to do some unlearning—and unlearning, unfortunately, is very hard to do.