GPatterson -- that's a very good point. I've run into that exact problem before. I was going to address that in part 2....but as a preview let me just say that the ISNULL() has to resolve to an impossible value, or else the full join will over-join. I had an ISNULL(<field>,0) once where the field had a possible value of 0, and this caused a problem.

Thanks for your input.