<Tr> <Td> Primary </Td> <Td> The key that is selected as the primary key . Only one key within an entity is selected to be the primary key . This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities . When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables together when selecting data . </Td> </Tr> <Tr> <Td> Alternate </Td> <Td> A non-primary key that can be used to identify only one row in a table . Alternate keys may be used like a primary key in a single - table select . </Td> </Tr> <Tr> <Td> Foreign </Td> <Td> A unique key that has migrated to another entity . </Td> </Tr> <P> At the most basic definition, "a key is a unique identifier", so unique key is a pleonasm . Keys that are within their originating entity are unique within that entity . Keys that migrate to another entity may or may not be unique, depending on the design and how they are used in the other table . Foreign keys may be the primary key in another table; for example a PersonID may become the EmployeeID in the Employee table . In this case, the EmployeeID is both a foreign key and the unique primary key, meaning that the tables have a 1: 1 relationship . In the case where the person entity contained the biological father ID, the father ID would not be expected to be unique because a father may have more than one child . </P>

An entity that does not require a relationship to another entity for identification is called