Foreign key

primary key into another table. Since the primary key can be used to look up the information in another table, this information does not need to be included - only the primary key.

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 number 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 assumes its existence as a primary key somewhere else. Improper foreign key/primary key relationships are the source of many database problems.