If you can’t beat them, join them
Just as Boyce/Codd normal form is defined in terms of functional dependencies, so fifth normal form (5NF) is defined in terms of join dependencies (JDs); as noted in Chapter 4, in fact, 5NF is the normal form with respect to JDs, just as BCNF is the normal form with respect to FDs. And the treatment of these ideas in this part of the book therefore parallels the treatment of BCNF and FDs in Part II. In other words, I plan to treat the material both formally, in Chapter 10, and informally in the present chapter.
Let me immediately add that although 5NF is indeed “the” normal form with respect to JDs, this state of affairs shouldn’t be taken to mean that 5NF is the ultimate goal of the normalization process. Au contraire, in fact: There are at least two other normal forms that have a better claim to that title, as we’ll see in Chapter 13. From a pedagogical point of view, however—as well as from a historical one—I think it’s desirable to discuss 5NF in detail first. (I mention this point simply in order to avoid giving a false impression; one of my reviewers felt I should have presented the material in a different sequence, but I don’t agree.)
Now, in previous writings I’ve tended to regard JDs as if they were just a generalized kind of FD. I now think this perception is wrong, or at least misleading; I now think it’s better to regard JDs as a completely different phenomenon. Of course, FDs and JDs are both ...