Candidate key

Any column or group of columns which can uniquely identify a data set, and can therefore be considered for use as a primary key. It is always NOT NULL (i.e. must not be left undefined), and unique, which means that the values in theses column(s) must never change. It is therefore unadvisable to use columns such as surnames, telephone numbers and the like. Candidate keys do not have to be used as primary or foreign keys, and the automatically generated primary key ID remains the preferred method.