My soul, sit thou a patient looker-on;
Judge not the play before the play is done;
Her plot hath many changes; every day
Speaks a new scene; the last act crowns the play.
A relational approach to SQL: That’s the theme, or one of the themes, of this book. Of course, to treat such a topic adequately, I need to cover relational issues as well as issues of SQL per se—and while this remark obviously applies to the book as a whole, it applies to this first chapter with special force. As a consequence, this chapter has comparatively little to say about SQL as such. What I want to do is review material that for the most part, at any rate, I hope you already know. My intent is to establish a point of departure, as it were: in other words, to lay some groundwork on which the rest of the book can build. But even though I hope you’re familiar with most of what I have to say in this chapter, I’d like to suggest, respectfully, that you not skip it. You need to know what you need to know (if you see what I mean); in particular, you need to be sure you have the prerequisites needed to understand the material to come in later chapters. In fact I’d like to recommend, politely, that throughout the book you not skip the discussion of some topic just because you think you’re familiar with that topic already. For example, are you absolutely sure you know what a key is, in relational terms? Or a join?
 There’s at least one pundit who doesn’t. The following is a direct quote from a document purporting (like this book!) to offer advice to SQL users: “Don’t use joins ... Oracle and SQL Server have fundamentally different approaches to the concept ... You can end up with unexpected result sets ... You should understand the basic types of join clauses ... Equijoins are formed by retrieving all the data from two separate sources and combining it into one, large table ... Inner joins are joined on the inner columns of two tables. Outer joins are joined on the outer columns of two tables. Left joins are joined on the left columns of two tables. Right joins are joined on the right columns of two tables.”