SQL and Relational Theory, 2nd Edition

Chapter 4. No Duplicates, No Nulls

I haven’t even mentioned yet the way the silly notions
Discussed so far interreact and lead us into oceans
Of complication and despond and general distress.
Are two nulls equal (duplicates)?  I fear, both NO and YES.

—Anon.: Where Bugs Go

In the previous chapter, I said the following (approximately):

• Relations never contain duplicate tuples, because the body of a relation is a set (a set of tuples) and sets in mathematics don’t contain duplicate elements.

• Relations never contain nulls, because the body of a relation is a set of tuples, and tuples in turn never contain nulls.

I also suggested that since there was so much to be said about these topics, it was better to devote a separate chapter to them. This is that chapter. Note: By definition, the topics in question are SQL topics, not relational ones; in what follows, therefore, I’ll use the terminology of SQL rather than that of the relational model (for the most part, at any rate).