SQL and Relational Theory, 2nd Edition

TYPES AND RELATIONS

Data types (types for short) are fundamental to computer science. Relational theory in particular requires a supporting type theory, because relations are defined over types; that is, every attribute of every relation is defined to be of some type (and the same is true of relvars too, of course). For example, I’m going to assume throughout this book that attribute STATUS of the suppliers relvar S is defined to be of type INTEGER. Under that assumption, every relation that’s a possible value for relvar S must also have a STATUS attribute of type INTEGER—which means in turn that every tuple in such a relation must also have a STATUS attribute that’s of type INTEGER, which means in turn that the tuple in question must have a STATUS value that’s an integer.

I’ll be discussing such matters in more detail later in this chapter. For now, let me just say that—with certain important exceptions, which I’ll also be discussing later—a relational attribute can be of any type whatsoever, implying among other things that such types can be arbitrarily complex. In particular, those types can be either system or user defined. In this book, however, I don’t plan to say much about user defined types as such, because:

• The whole point about user defined types (from the point of view of the user who is merely using them, that is, as opposed to the user who actually has the job of defining them) is that they’re supposed to behave just like system defined types anyway.

• Comparatively few ...