Is it fine to have foreign key as primary key?

Yes, it is fine to have a foreign key as a primary key in certain scenarios, especially when implementing one-to-one relationships in a relational database. A foreign key as a primary key ensures that the record in the referencing table is uniquely linked to a record in the referenced table, maintaining referential integrity. This is commonly used in cases where the child table extends the parent table with additional attributes. However, it is important to design the database schema carefully to avoid complications and ensure efficient queries. Proper indexing can also improve performance in such cases.