Describe the primary key functions?

Primary Key : The primary key is an attribute or a set of attributes in which uniquely identify a specific example of an entity. Every entity in the data model must have a primary key whose values uniquely recognize instances of the entity.

To qualify as a primary key for an entity, an attribute must have the subsequent properties:
* It must have a non-null value for each instance of the entity.
* The value must be unique for each instance of an entity
* The values must not change or become null during the life of each entity instance.