ben I sense your frustration. I've personally felt it many times in the past. I am only recently coming to the conclusion that difficulties with primary and secondary data sources are best solved in the database (using Views) before connecting to Tableau.
Caveat: This is only my personal opinion, and is not intended as a "best practice" or "best way" to do something in Tableau.
Argh, trying to avoid adding more complexity by including a DB step as it's something we'd like to update fairly frequently, but thanks for the tip. Anyone else on if this issue is a limitation in Tableau or something I'm doing wrong?