We've covered several programming languages now, but we haven't really gotten into database development yet. Although NoSQL is hot, SQL (structured query language) isn't going away. Different databases (such as Oracle, MySQL and PostgreSQL) use different variations, but the idea is to have a unified system so that a developer can move from database to database with a minimum of relearning.
If you're not quite sure yet what a relational database is or why someone would use one, check out our guide to database terminology. Otherwise, let's dive into some books for beginners, and one for experienced database developers.
Practical PostgreSQL from O'Reilly Media is designed to introduce the SQL beginner to PostgreSQL. The book is focused on Postgres running on Red Hat, but has many lessons applicable to other environments.
W3 Schools SQL Tutorial
Love 'em or hate 'em, W3 Schools has tutorials on nearly every Web development subject, and SQL is no exception.
Oracle Database 10g Express Edition Tutorial
This Oracle Database 10g Express Edition tutorial introduces Oracle's SQL database using its free version.
Build Your Own Database Driven Web Site Using PHP & MySQL
If you're planning on learning MySQL along with PHP, you can check out Sitepoint's Build Your Own Database Driven Web Site Using PHP & MySQL by Kevin Yank. The first 168 pages are available as a free PDF if you're willing to fork over your e-mail address. However, you can find much of the material on the Web in this series of tutorials by Yank.
You can find more resources for learning PHP here.
SQL Performance Explained
For the more advanced practitioner, there's SQL Performance Explained. In the words of the publisher it "It avoids unnecessary details about database internals but highlights the one topic that is most often neglected: proper indexing." It's available as an e-pub, or as a continually updated website called Use The Index, Luke.