SQL and Relational Theory, 2nd Edition

CONCLUDING REMARKS

It’s a common misconception that the relational model deals only with rather simple types: numbers, strings, perhaps dates and times, and not much else. In this chapter, I’ve tried to show among other things that this is indeed a misconception. Rather, relations can have attributes of any type whatsoever (other than as noted in just a moment)—the relational model nowhere prescribes just what those types must be, and in fact they can be as complex as you like. In other words, the question as to what types are supported is orthogonal to the question of support for the relational model itself. Or, less precisely but more catchily: Types are orthogonal to tables.

I also remind you that the foregoing state of affairs in no way violates the requirements of first normal form—first normal form just means that every tuple in every relation contains a single value, of the appropriate type, in every attribute position. Now we know those types can be anything, we also know all relations are in first normal form by definition.

Finally, I mentioned in the introduction to this chapter that there were certain important exceptions to the rule that relational attributes can be of any type whatsoever. In fact, there are two (both of which I’ll simplify just slightly for present purposes). The first is that if relation r is of type T, then no attribute of r can itself be of type T (think about it!). The second is that no relation in the database can have an attribute of any pointer ...