Some starting points based on existing Oracle documentation
[url http://www.oracle.com/technetwork/indexes/documentation/index.html]Oracle Documentation
SQL - [url http://docs.oracle.com/cd/E11882_01/appdev.112/e10766/toc.htm]2 Day Developer's Guide
XML DB - [url http://docs.oracle.com/cd/E11882_01/appdev.112/e23094/xdb01int.htm#ADXDB0100]XML DB Developer's Guide
If you are going for the basics of XML, here is a place to start at [url http://www.w3schools.com/xml/]XML Tutorial
You can also just google for tutorials on both SQL and XML and find the basics as well.