How can I avoid NULLs in my database, while also representing missing data?

Question

In SQL and Relational Theory (C.J. Date, 2009) chapter 4 advocates avoiding duplicate rows, and also to avoid NULL attributes in the data we store. While I have

Answer 1

Quite simply by storing only the known information - in other words the Closed World Assumption. Aim to be in at least Boyce Codd / Fifth Normal Form and you won't go far wrong.