It's worth taking a few moments to examine the question of why, as I claimed earlier, you as a database professional need to know the relational model. The reason is that the relational model isn't product-specific; rather, it is concerned with principles. What do I mean by principles? Well, here's a definition (from Chambers Twentieth Century Dictionary):
The point about principles is this: they endure. By contrast, products and technologies (and the SQL language, come to that) change all the time but principles don't. For example, suppose you know Oracle; in fact, suppose you're an expert on Oracle. But if Oracle is all you know, then your knowledge is not necessarily transferable to, say, a DB2 or SQL Server environment (it might even get in the way of your making progress in that new environment). But if you know the underlying principles in other words, if you know the relational model then you have knowledge and skills that will be transferable: knowledge and skills that you'll be able to apply in every environment and that will never be obsolete.
In this book, therefore, we'll be concerned with principles, not products, and foundations, not fads. Of course, I realize that sometimes you do have to make compromises and trade-offs in the real world. For one example, sometimes you might have good pragmatic reasons for not designing the database in the theoretically optimal way (an issue I discuss in Chapter 7). For another, consider SQL once again. Although it's certainly possible to use SQL relationally (for the most part, at any rate), sometimes you'll find because existing implementations are so far from perfect that there are severe performance penalties for doing so . . . in which case you might more or less be forced into doing something not "truly relational" (like writing a query in some weird and unnatural way in order to get the implementation to use an index). However, I believe very firmly that you should always make such compromises and trade-offs from a position of conceptual strength.That is:
The following quote which is attributed to Leonardo da Vinci (1452-1519) and is thus some 500 years old! sums up the situation admirably:
(OK, I added the italics.)