This article demonstrates how to use a subreport to retrieve data on the same report from two different databases.  You can expand on this sample by creating multiple subreports in a single report or use a subreport to run separate and distinct queries against the same database but display the data in a single report.  There are many instances where subreports can give you the flexibility you need when a simple SQL query cannot accomplish your goal.  Good luck on your project and happy coding.