PostgreSQL's documentation is an excellent reference - you can learn SQL, and learn it well, just from the manual. It is pretty dry, though. OpenBSD's man page

PostgreSQL's documentation is an excellent reference - you can learn SQL, and learn it well, just from the manual. It is pretty dry, though. OpenBSD's man pages are also excellent reference documents. One really great thing is that they document all possible behaviors, which is disturbingly rare for (e.g.) Python libraries.