<grin>Well, you're right on with your description of the requested task. (Have you spent some time in healthcare?) I had started out with a database, but the potential of 10 diagnosis columns and thinking about splitting the data into tables and writing the query seemed a bit laborious. I've not used the NOT EXISTS syntax before--I'll have to look into that for future reference. Thanks.
You don't have to work in healthcare for this kind of observation. It's a very common 'goose-chase' scenario that people do when they would really be better off using a database.