A value that represents a reference to a tuple a row in a table containing the matching candidate key value(The problem of ensuring that the database does not include any invalid foreign key values is known as the referential integrity problem. The constraint that values of a given foreign key must match values of the corresponding candidate key is known as a referential constraint. The relation (table) that contains the foreign key is referred to as the referencing relation and the relation that contains the corresponding candidate key as the referenced relation or target relation. (In the relational theory it would be a candidate key, but in real database management systems (DBMSs) implementations it is always the primary key))
