It depends a bit on how they are related. Database views can be used, but you can also use related list conditions on your report. 

The % thing may be a bit tricky. Within normal reporting, you may need to start using a custom checkbox or something like that (true when mapped). Or use a bar chart with different data sources to show you the total and the ones that are mapped (through related list conditions) and the data below the graph will show you percentages.