"Books Online" - Online

The most up-to-date version of the most important SQL reference material Books Online can be found here. Books Online is the definitive SQL reference - very useful if you are unsure of the correctness of other material such as books.

The most recent download-able version (July 2006 at the time of writing) can be found here. The website version is always the most up-to-date version - the download versions are simply point-in-time snapshots of the material on the website.