Data Base Design by Gio Wiederhold

By Gio Wiederhold

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For example, the predicate for relvar S is what it is, regardless of whether it’s expressed in English or Spanish or whatever. For simplicity, however, I’ll assume in what follows that a predicate is indeed just a sentence per se, expressed (usually) in some natural language. ) Finally, I’ve now explained what I mean by the term; however, you should be aware that, the previous paragraph notwithstanding, there seems to be little consensus, even among logicians, as to exactly what a predicate is.

However, while this usage on SQL’s part isn’t exactly incorrect, it does usurp a very general term—one that’s extremely important in database contexts—and give it a rather specialized meaning, which is why I prefer not to follow that usage myself. Second, I should explain in the interest of accuracy that a predicate isn’t really a natural language sentence as such, like the ones shown above; rather, it’s the assertion made by such a sentence. For example, the predicate for relvar S is what it is, regardless of whether it’s expressed in English or Spanish or whatever.

To sum up: There’s a logical difference between relation values and relation variables. For that reason, I’ll distinguish very carefully between the two from this point forward—I’ll talk in terms of relation values when I mean relation values and relation variables when I mean relation variables. However, I’ll also abbreviate relation value, most of the time, to just relation (exactly as we abbreviate integer value most of the time to just integer). And I’ll abbreviate relation variable most of the time to relvar; for example, I’ll say the suppliers-and-parts database contains three relvars (more specifically, three “real” or base relvars, so called to distinguish them from “virtual” relvars or views).

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