I'd like to add something here... I think a few people have touched on it, but it's pretty important.

If you are pulling out a very small amount of records, say, just one or two, then using Except would probably be the better way to go, if you can filter to this level in the except part of your clause.

If you are, however, pulling out a rather large set of data, the left outer joins will be the better choice.

At least, that's what I generally find with nested queries rather than joined queries.

If you can filter down to only a few records early in the game, then nesting is good. Otherwise, it makes your DB take too much of a time hit to query all the records twice, rather than things like hash matches.