Though I know that I have used this logic myself and simplified my queries because I knew that a foreign key guaranteed the behavior I desired, in retrospect, I question if that was my best strategy. By including the "exists" I am, first of all, making clear of what my target data should consist which may help someone reading my code to better understand my intentions. Most importantly, my query will return the correct data even if someone removes the foreign key constraint and adds invalid data into the table. While I shouldn't have to worry about that possibility, in the real world, databases are not always locked down and strictly controlled by a conscientious DBA and the possibility of a change like a constraint going missing is hardly unheard of. In such an environment there are still plenty of paths to catastrophe but a little redundancy can be fairly low-cost insurance against some of the more common screw-ups, particularly if the optimizer reduces any performance impact.

- Les