This looks cool, I'll have to check this book out. For people who are interested, this is from Foundations of Databases (http://www.amazon.com/Foundations-Databases-The-Logical-Leve...) and is available for free at the root url from jamii's link.

edit: realized it's a referral link, it's not mine, it's because I got there through Rich Hickey's list of books that influenced clojure: http://www.amazon.com/Clojure-Bookshelf/lm/R3LG3ZBZS4GCTH