What’s formal is normal What’s not so is not And if normal is formal, Informal is what?
Now I want to step back, take a deep breath as it were, and consider FDs and BCNF all over again—but this time I want to do it properly (with apologies for the small amount of repetition involved). As you’ll quickly see, the treatment in this chapter is rather more abstract than that in the previous one; it shouldn’t be difficult to follow, if you’re fully comfortable with the material of that previous chapter, but it’ll certainly be more formal. For that reason, I don’t want you to look at this chapter at all until you’ve absorbed everything in the previous one. (Of course, that shouldn’t be hard to do, since most of what was in that chapter was surely familiar to you anyway.)
One general point up front: Since BCNF is the normal form with respect to FDs, I won’t have anything to say in this chapter regarding 2NF or 3NF (or indeed 1NF). As I’ve more or less said already, 2NF and 3NF just aren’t all that interesting in themselves.
In this section I simply give definitions, with little by way of further elaboration, of a few familiar but absolutely fundamental concepts—definitions that are rather more precise than the ones typically found in the literature (as well as being more precise, in some cases, than ones given earlier in this book). Production of examples to illustrate the definitions is left as an exercise.