You may find it useful to install the free MySQL workbench tool and download this file.
http://www.examulator.com/er/moodle_31_erd.mwb
That way you can see all the tables and not rely on my possibly arbitrary choices on what to include in those diagrams.
If you are not running Moodle 3.1 you may find your version among these
http://www.examulator.com/er/archive.html
Though the database structure does not tend to change wildly from one release to another.