When I run the two queries (outer join vs NOT EXISTS),

the outer join query runs in 4-9ms (69 logical IO n the temp tables and 0 scans and 0 logical IO on the work table)

and the not exists query runs in 28-32ms (109 logical IO on the temp tables and 50 scans and 5079 logical IO on the work table).

It looks like the #CourseCompleted table is scanned 50 times in the NOT EXISTS query, whereas only 30 times in the outer join query.

I wonder if these results hold over larger data sets.