The Helix ALM Server database ERD is available here.
The schema uses the following names for item types: defects, requirements, requirement documents, test cases, and test runs. Keep in mind that your organization may have renamed these items in Helix ALM. The default terminology for 'defect' changed to 'issue' in TestTrack 2012.0, but the database schema still uses 'defect'.
The easiest way to search is on the Table details page. Use the browser search to find the information you need.
If you are looking for an ERD for an older version or a PDF, visit the knowledgebase.
You can find more Helix ALM documentation on our web site.