Take another look at Figure 1-1, the suppliers-and-parts database, in Chapter 1. That figure shows three relations: namely, the relations 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 relations appearing in their place. In other words, S, P, and SP are really variables—relation variables, to be precise—and just like variables in general, they have different values at different times. And since they’re relation variables specifically, their values at any given time are, of course, relation values.
As a basis for examining these ideas further, consider Figure 2-1 below. That figure shows (a) on the left, a very much reduced version of the shipments relation from Figure 1-1; (b) on the right, the relation that results after a certain update has been performed. Using the terminology of the previous paragraph, then, we can say, that (a) on the left of the figure we see the relation value that’s the value of relation variable SP at some particular time T1; (b) on the right, we see the relation value that’s the value of that same relation variable at some presumably later time T2, after an additional tuple has been inserted.
Figure 2-1. Relation values and variables–an example
So there’s a logical difference between relation values and relation variables. ...