A very complicate TSQL sample... in my opinions: http://www.vsj.co.uk/articles/display.asp?id=540

Even more implementation - http://lambda-the-ultimate.org/node/2291

which introduce some interesting function in vendor specific SQL, like Oracle MODEL - http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14223/sqlmodel.htm#sthref1855