Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In the context of relational databases, 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", "national_insurance_number", and so on. In this case, both "employee_id" and "national_insurance_number" 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, "national_insurance_number" was chosen as the primary key, "employee_id" would become the alternate key.

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