I have released a version of the Moodle database entity relationship (ER) diagram generated from the
Moodle 2.9 code which can be found at
Although the tables in core Moodle do not change a much between releases this diagram is a little more ambitious than the previous ones. It incorporates relationships missing from the release code which can be found in this bug report
https://tracker.moodle.org/browse/MDL-49795
Also the SQL code that was used to reverse engineer the diagram included field comments. When Moodle is installed it will normally throw away all field level comments. I updated the code so it kept the comments. Due to some quirks the comments may be truncated but I decided some information is better than none.
This diagram is probably most helpful if you download the *.mwb file and load it up in MySQL workbench (a free cross platform tool) but I hope the graphics/png files are also useful. Check out the component diagrams as well at