A new documentation is available for Squash TM 2.0 and later versions, check it here
To help you working with the database of Squash TM, you will find below the physical data models (PDM) of the version 1.20.x.
This is a simplified version, in the way that only the significant tables are represented.
The PDM is available in PNG format, and in PDF format (1 or 6 sides) :