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.
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.
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