Alternate key
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An alternate key is any candidate key which is not selected to be the primary key.
For example, a relational database with a table "employee" could have attributes like "employee_id", "ssn", and so on. In this case, both "employee_id" or "ssn" serves as a unique identifier for a given employee, and could thus argueably be used for a primary key. Hence, both of them are called "candidate keys", but this term only applies when no primary key is chosen. If, for example, "ssn" was chosen as the primary key, "employee" would become the alternate key.
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