Description:
- An instance of an entity
Identifier:
- Each ER model entity instance have identifier, which is attribute of the instance.
- Examples include ItemNumber identifying an instance of ITEM, and SocialSecurityNumber identifying an instance of EMPLOYEE.
- Identifiers may be unique or non-unique:
- Unique identifiers identifies one, and only one, entity instance
- Non-unique identifiers identifies a set in instances
- An example is EmployeeName is an example as there may be more than one person with that name