Database in Depth

Chapter Three. Tuples and Relations

From the first two chapters you should have gained a pretty good understanding of what tuples and relations are, at least from an informal point of view. Now I want to define those concepts more precisely, and I want to explore some of the consequences of those precise definitions. Perhaps I should warn you that the definitions can look a little daunting, but that's not unusual with formal definitions; the tuple and relation concepts themselves are quite straightforward, once you've struggled through the formalism, and you should be ready to do that by now because the terminology, at least, should be reasonably familiar to you.

What's a Tuple?

Is this a tuple?

Figure Three-1. 

Well, no, of course it isn't—it's a picture of a tuple, not a tuple as such (and note that for once I've included the type names as well as the attribute names in that picture). As we saw in Chapter 1, there's a difference between a thing and a picture of a thing, and that difference can be very important. For example, tuples have no left-to-right ordering to their attributes, and so the following is an equally good (or bad?) picture of the very same tuple :

Figure Three-2. 

Thus, while I'll certainly be making use of pictures like these in the sections to follow, please keep in ...