Seperation of JOIN conditions and WHERE conditions, to allow you to perform an outer join and a filter within a single query.
(Granted, there are ways around it, but it would be nicer to be able to make one query do the job. See https://bobj-board.org/t/99872 and https://bobj-board.org/t/100493)
HuwD 
(BOB member since 2007-04-19)