This book began life as a comparatively short chapter in a book called Database in Depth: Relational Theory for Practitioners (O’Reilly, 2005). That book was superseded by SQL and Relational Theory: How to Write Accurate SQL Code (O’Reilly, 2009), where the design material, since it was somewhat tangential to the main theme of the book, ceased to be a chapter as such and became a (somewhat longer) appendix instead. I subsequently began work on a second edition of this latter book.^[1] During the course of that work, I found there was so much that needed to be said on the subject of design that the appendix threatened to grow out of all proportion to the rest of the book. Since the topic was, as I’ve indicated, rather out of line with the major emphasis of that book anyway, I decided to cut the Gordian knot and separate the material out into a book of its own: the one you’re looking at right now.