Association Aggregation Composition Interview Questions

Association means two objects know each other, like a teacher and a student. Neither owns the other's lifetime. Aggregation is has-a, but the part can live on its own — a department has professors, and professors still exist if the department is dissolved. Composition is has-a with ownership — a house has rooms; if I destroy the house, the rooms go with it. In a database that often means cascade delete. I use this when modeling, not as trivia. If the lifetime is independent, I do not draw a filled composition diamond.

Association: Student and Course, neither owns the other's life. Aggregation: Department has Professors who can exist elsewhere. Composition: House and Room — destroy the house, rooms go. I am sharp on composition equals owned lifecycle. That maps to cascade delete.

Composition often means cascade delete: order_lines die with order. Aggregation might be a nullable FK. Association is a join table or an FK without cascade. I would not cascade-delete customers when I delete a department.

If the engine is destroyed with the car, composition. If I can move the engine to another car, aggregation. Real engines are often aggregation. I ask about lifecycle, not the English word has.

Almost never. Composition is exclusive ownership. Students and courses are a junction table. A filled diamond on many-to-many is a UML smell.