Sometimes no single column is unique, but a combination is, like one student enrolling in many courses. A composite key makes the pair, such as (student_id, course_id), the identifier. Reach for it in junction tables for many-to-many relationships. Do not keep adding columns just to be safe: a key must be minimal, and if the business rule allows repeats, like retaking a course, the key has to change rather than grow blindly.