A constraint is a data rule or restriction that is enforced within the database rather than at application or object level.
The following constraint types are available in Oracle, as can be confirmed by selecting from the DBA_CONSTRAINTS view:
|Type Code||Type Description||Acts On Level|
|C||Check on a table||Column|
|O||Read Only on a view||Object|
|R||Referential AKA Foreign Key||Column|
|V||Check Option on a view||Object|
Users may need to write database triggers to enforce complicated constrains that cannot be handled with the above declarations.
 Also see
|Glossary of Terms|