How can we implement “A student can enroll in up to 4 units in a semester ”

Question

I have a practice problem on Database implementation and am confused. For this database, I need to store enrollment information on students. Information such as which units a student has enrolled into. The relationship between Units and Students is such that a student can enroll in up to 4 units per semester but a unit can be enrolled into by many students. My problem is, how can I implement this limitation of a student enrolling in up to only 4 units per semester in mysql using either CHECKs or triggers?

If it was one to many relationship between Student and Units that would have been straight forward but here we have a one to four(?) relationship?

Answer 1

I see at least 2 possibilities.

First - BEFORE INSERT / UPDATE triggers which checks the amount of enrolls per student and forbids insertion / updation if the restriction fails. UPDATE trigger needed for to restrict errorneous data while updating with student ID change.

Second - add enroll_number column which is NOT NULL, ENUM('1','2','3','4') and UNIQUE in the combination with student ID.

I am not considering the option where the table contains 4 fields for 4 separate enrolls - this is denormalized structure and bad practice. Formally the same for a variant when all enrolls are stored in one column of some serialized datatype (JSON, XML).