Consider Figure $2\mathrm{.}1.$ In addition to constraints relating the values of columns in one table to columns in another table, there are also constraints that impose restrictions on values in a column or a combination of columns within a table. One such constraint forces that a column or a group of columns must be unique across all rows in the table. For example, in the STUDENT table, the StudentNumber column must be unique $($to prevent two different students from having the same StudentNumber$).$ Identify the column or the group of columns in the other tables that must be unique across all rows in the table?