We have created a powerful sample application in WaveMaker that provides an introduction to Hibernate Queries. In the application, each Hibernate query concept is described (inner join, outer join, group by, etc) and then you can test it out and see the results displayed live.
The application itself is attached to this post. If you get inspired, create a few HQL queries of your own and submit them back to the community!
There is also a detailed Hibernate Tutorial on the Wavemaker Wiki.
Finally, the wiki also has a white paper we wrote on using WaveMaker as a tool for Java developers to automate the tedious process of Hibernate mapping.