Foreign key
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A foreign key (FK) is a field in a database record that points to a key field of another database record in another table. Usually a foreign key in one table refers to the primary key (PK) of the other table. This way references can be made to link information together and it is an essential part of database normalization.
For example, a person sending an e-mail need not include the entire text of a book in the e-mail. Instead, they can include the ISBN of the book, and interested persons can then use the number to get information about the book - or even the book itself. The ISBN is the primary-key of the book, and it is used as a foreign-key in the e-mail.
Note that using a foreign key often assumes its existence as a primary key somewhere else. Improper foreign key/primary key relationships are the source of many database problems.