An entity may have one or more choices for the primary key. Collectively these are known as candidate keys. One is selected as the primary key. Those not selected are known as secondary keys.
For example, an employee has an employee number, a National Insurance (NI) number and an email address. If the employee number is chosen as the primary key then the NI number and email address are secondary keys. However, it is important to note that if any employee does not have a NI number or email address (ie: the attribute is not mandatory) then it cannot be chosen as a primary key.
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