Super Key

A super key is any set of one or more columns whose values uniquely identify each row in a table. It may contain extra columns that are not needed for uniqueness.

For example, if employee_id is unique, then {employee_id}, {employee_id, name} and {employee_id, name, email} are all super keys. A candidate key is a super key with no unnecessary columns, so every candidate key is a super key but not every super key is a candidate key.