Originally Posted by Old Pedant
Probably the LEFT JOIN answer is beyond where he is in the class or book, anyway.
Probably the instructor is expecting him to use a NOT IN( ) clause at this stage in the class.
That's one of the problems with giving answers to homework - the answer will almost certainly include code that is well beyond that so far covered in the course. Of course that makes it far easier for the teacher to tell who cheated since anyone using code that is beyond where they are in class would need to be able to explain exactly how that code works in order to prove that they really are ahead of the class and if they can't then it will be obvious they cheated. Most likely the teacher already knows who is ahead anyway and so wouldn't even need to ask.