One and Only One in ER Diagram

In ER diagram, the "one and only one" relationship refers to a relationship between two entities, where one entity is associated with exactly one instance of the other entity, and each instance of the other entity is associated with exactly one instance of the first entity. This relationship is also known as a "one-to-one" relationship.

Example

An example of a "one and only one" relationship in ER diagram would be between a Person entity and a Government ID entity. In this case, each person can have only one government ID, and each government ID can belong to only one person.

Person ──────(1:1)──────► Government ID

Implementation

In a database schema, a "one and only one" relationship can be implemented using foreign keys and unique constraints. In the above example, the Person entity would have a foreign key to the Government ID entity, and the Government ID entity would have a unique constraint on the foreign key column.

Conclusion

The "one and only one" relationship is an important concept in ER diagrams and database design. It represents a relationship where each instance of one entity is associated with exactly one instance of another entity, and vice versa. This relationship can be implemented using foreign keys and unique constraints in a database schema.