A foreign key, also called a foreign keyword, in a database table is a key from another table that refers to (or targets) a specific key, usually the primary key , in the table being used. A primary key can be targeted by multiple foreign keys from other tables. But a primary key does not necessarily have to be the target of any foreign keys.
It is possible to change the primary key in a table when the specific needs of the users change. 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. When the primary key in a table is changed, the set of associated foreign keys, if there are any, often changes as a result.