verified when a row is directly updated (for performance reasons,) and often implemented as implied INSERT or UPDATE triggers, integrity constraints could be violated by indirect action were it not for these limitations. Furthermore, otherwise-valid modifications to these records would then be prevented by the CHECK constraint. Some examples of dangerous constraints include: User-defined triggers can be used to work around these restrictions. Although similar in implementation, it is semantically clear that triggers will only be fired when the table is directly modified, and that it is the designer's responsibility to handle indirect, important changes in other tables; constraints on the other hand are intended to be "true at all times" regardless of the user's actions or the designer's lack of foresight. References SQL