Referential Integrity
The principle of ’Referential Integrity’:
- To maintain valid database content, all fkey values must match the unique primary key or object identifier of another tuple, or else have the reserved ‘null’ (unknown or undefined) value.
N-ary relations (N-way associations) can be implemented by a new associative entity, whose tuples contain exactly N fkeys (plus optional non-key attributes).
Most relations are binary (N = 2). Note that fkeys may refer to the same or different types.