Alternate key

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An alternate key (or secondary key) is any candidate key which is not selected to be the primary key (PK).

For example, a relational database with a table "employee" could have attributes like "employee_id", "bank_acct_no", and so on. In this case, both "employee_id" and "bank_acct_no" serve as unique identifiers for a given employee, and could thus arguably be used for a primary key. Hence, both of them are called "candidate keys". If, for example, "bank_acct_no" was chosen as the primary key, "employee_id" would become the alternate key.