TFS database schema is not documented for the reason that you are not supposed to use it directly. The reason being that database schema changes between versions of TFS and whatsoever you would have created might/would stop working after an upgrade or update. Please use TFS API instead.
Having said that, there are some blogs and guides that use database directly. Have a look at Grant Holiday's blog who has posted about using some queries to get some statistics e.g.
http://blogs.msdn.com/b/granth/archive/2009/10/23/tfs2010-sql-queries-for-tfs-statistics.aspx
Also, ALM Rangers reporting guide contains an appendix about Work Item Store Database schema
https://vsarreportguide.codeplex.com/downloads/get/764892
DISCLAIMER: As I said, the data model can and will change over time so be mindful of that.