A primary key, also called a primary keyword, is a key in a relational database that is unique for each record. It is a unique identifier, such as a driver license number, telephone number (including area code), or vehicle identification number (VIN). A relational database must always have one and only one primary key. Primary keys typically appear as columns in relational database tables.
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The choice of a primary key in a relational database often depends on the preference of the administrator. It is possible to change the primary key for a given database when the specific needs of the users changes. For example, the people in a town might be uniquely identified according to their driver license numbers in one application, but in another situation it might be more convenient to identify them according to their telephone numbers.
Compare foreign key.