The enrolments table records which student enrolled in which course. It has no constraints yet. A student enrols in a course once.

1. Is student_id alone a superkey of enrolments? Answer yes or no.
2. Make the pair student_id, course_id the primary key of enrolments. The database refuses it. Which student does the DETAIL line name?
3. The second enrolment of that pair is a mistake: keep the row with the earlier date and remove the other one. Then add the primary key.
4. Add a constraint named enrolled_from_2020 that refuses an enrolled_on date before 2020-01-01.