Factless Facts

A factless fact is where the fact does not store an actual numerical measure, the mere existance of a fact record indicates that an event has happened that you wish to track. The classic example of this would be an "Attendance" fact. If you had dimensions to record date, scheduled_course, instructor and delegate then you could create a fact table that held the permutations of these dimensions. From this you could evaluate the number of courses you run, the number of delegates, the number of courses by instructor etc.

I would never simply leave a factess fact as a bare collection of foreign key columns I would always add a dummy measure column which would be set to 1 which you would then sum.