The query optimiser should be dealing with that. If you're doing an outer join and checking for the key being NULL then it can easily see that the operation can

The query optimiser should be dealing with that. If you're doing an outer join and checking for the key being NULL then it can easily see that the operation can be short circuited. It's all implementation details. I've worked with databases where a NOT EXISTS would run a query for every source row where as an outer join would run through once doing the join and was therefore faster.