Ask students for other examples of relations (tables) that could have more than one candidate key. Some examples are: state data that contain both state abbreviation and state name and inventory data that contain both a tag number and a serial number.
Have students read the Note on page 31. Ask them how they feel about using Social Security number as a primary key. Also ask them for additional databases where Social Security number is being used as a primary key other than banks and places of employment. Some examples are: insurance company, doctor’s office, dentist’s office.