A SQL Tutorial
Not going to say much in this post. RDBMS is more or less a ubiquitous technology which everyone uses without ever realizing. With more and more embedded systems starting to run operating systems, a database is one of the most component that the OS can provide support for (though technically i guess, you could compile SQLite for your bare bone platform). Most developers at some point or another will have to get their hands dirty using SQL to access the RDBMS. So, here is a very good tutorial by Mr. Philip Greenspun which will teach you how to use SQL in general.
Have a good time.