Foreign keys are used to associate data between tables. For instance, a foreign key of REP in the CUSTOMER_TBL points to the primary key EMPL_ID in the EMPLOYEE_TBL.

> A foreign key is one or more columns in a table whose values reference values in another table.
> A foreign key in the child table establishes a relationship to the primary key in the parent table.
> The matching foreign key and primary key can have different names.
> If this relationship is enforced, then this is called referential integrity.
> If the relationship is not enforced and there is a foreign key in a child table that does not match
the primary key of the parent table, then this is called an orphan row.
> Unlike primary keys, foreign keys can be null.
> Foreign keys are not always unique in their table.
> A foreign key can reference the primary key of the same table. For instance, the MANAGER of
an employee can reference the EMPLOYEE ID of the manager.