What SQL learning and reference resources are available online?

SQL online resources are not nearly as plentiful as many other areas, but here is a good basic tutorial: Introduction to Structured Query Language The page additionally has a number of links to other resources.

Also, and I hope it is legal, Teach Yourself SQL in 21 Days, Second Edition is online. And, of course, vendor manuals provide reference materials and some examples. See Where can I find online documentation for database xyz?