From c2.com under DatabaseBestPractices: I am bothered by (1:1)-to-(0:1) relationships. It creates lots of skinny tables, more joins, and creates change-problems. For example, if managers have certain fields only for managers, but later *all* employees get some of the same perks, then moving those fields back into Employees instead of Managers creates headaches. Thus, I suggest keeping it all together.