I’ve now covered all of the operators I want to cover in this chapter. As I’ve more or less said already, however, not all of those operators are primitive—some of them can be defined in terms of others. One possible primitive set is the set {restrict, project, join, union, difference}; another can be obtained by replacing join in this set by product. Note: You might be surprised to see no mention here of rename. In fact, however, rename isn’t primitive, though I haven’t covered enough groundwork yet to show why not (see Exercise 7.3 in Chapter 7). What this discussion does show, however, is that there’s a difference between being primitive and being useful! I certainly wouldn’t want to be without our useful rename operator, even if it isn’t primitive.