1
The database state consists of three relations: student (primary key: register number, foreign key: D number), department (primary key: D number), and instructor (primary key: teacher ID / TID, foreign key: D number).
2
Four types of constraints are checked during updates: Domain constraint, Key constraint, Entity integrity constraint (primary key cannot be null), and Referential integrity constraint (foreign key must match an existing primary key or be null).
3
Insert operations can potentially violate domain, key, entity integrity, and referential integrity constraints. Enforcement options include rejecting the insertion, modifying values to valid ones, or inserting matching parent tuples.
4
Delete operations typically only risk violating referential integrity constraints if other tuples reference the deleted tuple. Enforcement options include rejecting the deletion or cascading deletes.
5
Modify/Update operations on foreign keys risk violating referential integrity if the new value does not exist in the referenced parent relation.