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

You would just get the amount of rows returned from this example query:

SELECT COUNT(*) FROM enrollment WHERE student_id = 'XYZ' AND subject = 'ABC' AND academic_period = 'DEF'

If the amount of rows returned is greater than 0, they have already been enrolled in that subject in the academic period.