How do I enforce uniqueness in a table?

Question

For example, I have to program an enrolment table which has:

1. enrolment_id
2. academic_period
3. results
4. student_id
5. subject
<

Answer 1

Create a unique constraint on academic_period, student_id, subject
I'll let you work out the actual SQL

Note: this is correctly your natural key: enrolment_id is a surrogate added at implementation time because of the database architecture. It isn't mandated in any design or modelling process