Entity relationship diagram (ERD) for Moodle 3.1 is released recently by Marcus Green (UK Based teacher and Moodle developer) which shows the relationships of entity sets stored in the Moodle database.
The database schema or ERD diagram is an essential thing to look at from a novice point of view as it will will give you a clear picture of how complex Moodle aspects are like Quiz, Assignment, competency etc. The diagram will give you a fair idea about the Moodle Database design.
You can check out the web version of the ERD diagram here which is also available in PDF format. The diagram was generated by using the reverse engineering feature of the MySQL Workbench tool using a file containing the SQL code used to install Moodle. Check out the sample of the admin section ERD diagram below:
You can also check out the images of the individual components of diagram here. The diagrams, files and images are released under the GNU Public Licence or the Creative Commons License at http://creativecommons.org/licenses/by/4.0/
Check out the older archives of ERD diagrams for previous versions of Moodle here. For developers it is recommended to get start with the MySQL Workbench version of the diagram which can be downloaded here.