While still working on a post on the misuse of NULLs, I stumbled across the book "Applied Mathematics for Database Professionals" by Lex de Haan and Toon Koppelaars. Having some background in mathematics, set theory and a lot of background in logic, I found this book to be a gem. Not a polished gem that you might put on a neckless to give to your significant other, but more a priceless, raw, interestingly flawed one that you would hoard and hold on to for life.
This book is by far not an easy read, nor is it for the faint of heart. It's written very much like a textbook, and it is definitely not a "browseable book". It requires a basic understanding of set-based logic (which is provided), but then discusses most situations in a purely symbolic manner. It will take some work for anyone to get through - I have a background in this and I had to read carefully to make sure I followed everything properly. There are some chapters that are much more "readable", but they are someone dependent on understanding the previous, dryer chapters.
That being said, I fully believe that every database developer should understand the concepts this book examines. This is primarily a discussion of set theory and how it does or should fit into a relational database query language. It does have an Oracle slant, but the set theory part of it is pure set theory. There's also some interesting dicussions of NULL, three-value logic and even 4-value logic. Even though it contains 400pps of very dense, academic prose, I consider this a must read. In fact, this would make a great text to use for a custom query design/optimization course. Hmm...
BTW, I got my copy off of www.apress.com as an ebook for substantially less than the hard copy. If you're serious about being a SQL developer, this should be on your shelf (or hard drive). Depending on your experience, quite a lot of it might not be surprising or new, but even so it establishes much of what you may know intuitively in a formal manner. Check it out.