Understanding Functional Dependency

In explaining functional dependency to students, I have noticed in texts a mixture of two types of elements: intensional (or psychological or meaning) and extensional (patterns of repetition in the data). In this chapter I examine whether it is possible to consider functional dependency, in particular, in second and third normal forms, solely on an extensional basis. The Microsoft Access Analyzer utility seems to do so. I illustrate the mix of intensional and extensional elements in textbook definitions of functional dependency. I conclude that although in principle first, second and third normal form can be done solely by extensional means, in practice intensional considerations are indispensable. Finally, I discuss these questions with respect to the "higher order" normal forms, namely Boyce-Codd, fourth, fifth and Domain/Key normal form.