Keys (Cont.) K is a candidate key if K is minimal Example:{customer_name}is a candidate key for Customer,since it is a superkey and no subset of it is a superkey. Primary key:a candidate key chosen as the principal means of identifying tuples within a relation Should choose an attribute whose value never,or very rarely, changes. E.g.email address is unique,but may change Database System Concepts-5th Edition,Oct 5,2006 2.13 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 2.13 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Keys (Cont.) K is a candidate key if K is minimal Example: {customer_name} is a candidate key for Customer, since it is a superkey and no subset of it is a superkey. Primary key: a candidate key chosen as the principal means of identifying tuples within a relation Should choose an attribute whose value never, or very rarely, changes. E.g. email address is unique, but may change