This is the last kind of check we are going to explore. This kind of check has actually a quite limited use key, we have a column that should report some kind of key of our table, that is a value that should be able to unequivocally identify every record within our table. Let's think of the customer code within a customer list, we should be sure that no double customer ID is reported within our table since this would lead to doubts on which records should be considered as reliable for the purposes of retrieving data related to that given customer. As we have already seen, the uniqueness of our data can be checked by comparing the length of our column with the length of the vector composed of just the unique values.