The Moodle 3.1 Database Schema "reverse engineered" from the source xml files using
MySQL Workbench http://www.mysql.com/products/workbench. For more about Moodle see http://www.moodle.org
For more about the Moodle database structure see
http://docs.moodle.org/dev/Database_schema_introduction
Marcus Green 2016

A Mysql MWB File that can be modified. moodle_31_erd.mwb .This file contains additional diagrams that show some of the modules in more detail.
A pdf rendering moodle_31_erd.pdf. A set of images of individual components of the diagram components

How the diagram was created, Diagram FAQ. See here for an archive of  old versions

Moodle 3.0 ER Diagram