In this chapter I’ve given precise definitions for the fundamental concepts tuple and relation. As I said earlier, those definitions can be a little daunting at first, but I hope you were able to make sense of them after having read the first two chapters. I also discussed tuple and relation types, selectors, and comparisons, as well as a number of important consequences of the definitions; in particular, I briefly described the important relations TABLE_DUM and TABLE_DEE. And I discussed the SQL counterparts of all of these notions, where such counterparts exist. In closing, I’d like to stress the importance of the recommendations, in the section immediately preceding this one, regarding column naming in SQL. Later chapters will rely heavily on those recommendations.