What is a foreign key, and what is it used for?

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A foreign key is used to establish relationships among relations in the relational model.
Technically, a foreign key is a column (or columns) appearing in one relation that is (are)
the primary key of another table. Although there may be exceptions, the values in the
foreign key columns usually must correspond to values existing in the set of primary key
values. This correspondence requirement is created in a database using a referential
integrity constraint on the foreign key.