Question

Which of the following options are possible to implement referential integrity? (a) No Action (b) Set Null (c) Delete table (d) Update Cascade

Answer 1

Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship with non unique values with key.

It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.