In these questions we need to find “borrow” records that correspond to books of an author. The “borrow” records only contain the barcode, not the ISBN. So the “copy” table is needed to make the link between the book and borrowed copies of the book. Note the difference between the three questions. In the first we look at just one copy. In the second we look at all the copies of a book. In the third we look at all the books written by an author. Each time we need the “difference” operator, but on a different subquery.