Phillip Greesnspun's book Sql for Web Nerds is an excellent read. It explains the basic concepts of databases and SQL in a humorous and informative way. This web book focusses a primarily on Oracle, so that may be a bit of a dampener for those working with MySql or PostGres, but the concepts of RDBMS and SQL are pretty similar across databases. So as a worst case scenario, if you are not using Oracle, you may (at times) have to look for equivalent constructs in the database of your choice.
SQLZoo is a great tutorial for basic SQL commands.
