by Hugh Darwen. This book is a companion to An Introduction to Relational Database Theory and is intended to be read in parallel with that text, hereinafter referred to as “the theory book”. Although existing knowledge of SQL is not a prerequisite, this book is not intended to be used as an SQL primer. Rather, its aim is to investigate the extent to which SQL supports and adheres to the theory, how it does so, and where and how it departs from the theory.