[WhompingWindows]

Just about joins in particular. I get them now, but I struggled joining hospital tables together because they had around a dozen different ID keys that all meant something different - patient, provider, hospital, clinic, room, and these could vary geographically, so many of them were not "unique" and you needed to combine multiple ones. It's not so much the JOIN that was tough, but the universe of tables and keys we had to contend with at the hospital.