One-to-one relationships occur when an instance of an entity can have zero or one corresponding instance of another entity.
One-to-one entity relationships can be bidirectional (each entity is aware of the relationship) or unidirectional (only one of the entities is aware of the relationship). In the CUSTOMERDB database, the one-to-one mapping between the LOGIN_INFO and the CUSTOMERS tables is unidirectional, since the LOGIN_INFO table has a foreign key to the CUSTOMERS table, but not the other way around. As we will soon see, this fact does not stop us from creating a bidirectional one-to-one relationship between the Customer entity and the LoginInfo entity.
The source code for the LoginInfo entity, which maps to ...
