The Moodle 2.7 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. moodle2.7_erd.mwb .This file contains additional diagrams that show some of the modules in more detail.
A pdf rendering moodle27_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 2.7  ER Diagram