The Moodle 2.8 Database Schema "reverse engineered" from the installed database using
MySQL Workbench For more about Moodle see
For more about the Moodle database structure see
Marcus Green 2015

A Mysql MWB File that can be modified. moodle28_erd.mwb .This file contains additional diagrams that show some of the modules in more detail.
A pdf rendering moodle28_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 2.8  ER Diagram