The schema for the database is illustrated in Figure$2$ that depicts a relational database schema that includes an indication of primary and foreign key constraints in the manner discussed in class. Sample commands for defining the base tables for this schema can be downloaded from Moodle $($Example commands for creating the STUDENT$\_$COURSE database under the Assignments tab$).$ Note that the schema stores information about both ongoing and past classes for a course. Also note that no marks are recorded for any enrollment of an ongoing class, and that, for a past class, a mark is recorded for each of its enrollments. Finally, you may assume that each class has at least one enrollment.