How to create a missed-workshop-list?

Hi, 

I've got three tables for a report:

1.) Workshops (Title, Desciption)
2.) Workshop-Attendance (Title, Date, UserName)
3.) Users (Username, GivenName, LastName)

Relation between the tables are obvious (I hope).

I've been trying around for a few days now, without getting anywhere nearer to a solution:
How could I get a report showing all workshops users are still missing?
To be precise: output would need to be like (Username, Workshop) meaning user xyz has no entry in Workshop-Attendance for workshop abc.

Could somebody point me to an approach for this?

Thanks,

Marc