Application For each data family, CRUD operations will have to be available, with the following limitations: On courses and students data: neither courses nor students can be eliminated, if enrollment relationships exist. On companies: a company must have at least one operational office. The elimination of a company involves the removal of all the associated operational offices, after user’s confirmation. Operational offices (or related companies) cannot be eliminated if job placement relations involving them still exist. Enrollments: the same student cannot enroll multiple times in the same course. Enrollments cannot be eliminated if job placements still exist. The field resignation date is initially empty (NULL). Job placement: the same student can, if necessary, do multiple job placements with the same company, but in periods of time which are not overlapping.