Drupal 6 comes with a DB schema, which makes it possible now to programmatically draw a graph of the relationships among the tables.

Utilizing graphviz I played with the possibilities yesterday and here's a thumbnail of the result. Full-size versions can be found here.

The '.svg' and '.html' versions display a descriptive tooltip when you hover the mouse over fields.

Many tables are linked by "node.nid" or "user.uid" and in order to not clutter the display with too many "snakes" I colored "node.nid" links in green and "user.uid" ones in red. So wherever you see red or green lines don't bother following them with your eyes: you already know where they lead to.

Note that everything is done automatically. No tweaking is done by hand. The script uses drupal_get_schema() to learn about Drupal and a 'Makefile' is used to automate the generation of the various output files.