SQL and Relational Theory, 2nd Edition

RELATIONS vs. RELVARS

Now, it’s entirely possible that you already knew everything I’ve been telling you in this chapter so far; in fact, I rather hope you did, though I also hope that didn’t mean you found the material boring. Anyway, now I come to something you might not know already. The fact is, historically there’s been a lot of confusion over yet another logical difference: namely, that between relations as such, on the one hand, and relation variables on the other.

Forget about databases for a moment; consider instead the following simple programming language example. Suppose I say in some programming language:

    DECLARE N INTEGER ... ;

Then N here is not an integer. Rather, it’s a variable, whose values are integers as such—different integers at different times. We all understand that. Well, in exactly the same way, if I say in SQL—

     CREATE TABLE T ... ;

—then T is not a table: It’s a variable, a table variable or (as I would prefer to say, ignoring various SQL quirks such as duplicate rows and left to right column ordering) a relation variable, whose values are relations as such (different relations at different times).

Take another look at Figure 1-3, the suppliers-and-parts database. That figure shows three relation values—namely, the relation values that happen to exist in the database at some particular time. But if we were to look at the database at some different time, we would probably see three different relation values appearing in their place. In other words, S, P, and ...