Triggers Compared to Constraints
Constraints and DML triggers each have benefits that make them useful in special situations. The primary benefit of DML triggers is that they can contain complex processing logic that uses Transact-SQL code. Therefore, DML triggers can support all of the functionality of constraints; however, DML triggers are not always the best method for a given feature.
Entity integrity should always be enforced at the lowest level by indexes that are part of PRIMARY KEY and UNIQUE constraints or are created independently of constraints. Domain integrity should be enforced through CHECK constraints, and referential integrity (RI) should be enforced through FOREIGN KEY constraints, assuming their features meet the functional needs of the application.
DML triggers are most useful when the features supported by constraints cannot meet the functional needs of the application. For example:
FOREIGN KEY constraints can validate a column value only with an exact match to a value in another column, unless the REFERENCES clause defines a cascading referential action.
Constraints can communicate about errors only through standardized system error messages. If your application requires, or can benefit from, customized messages and more complex error handling, you must use a trigger.
DML triggers can cascade changes through related tables in the database; however, these changes can be executed more efficiently through cascading referential integrity constraints.
DML triggers can disallow or roll back changes that violate referential integrity, thereby canceling the attempted data modification. Such a trigger might go into effect when you change a foreign key and the new value does not match its primary key. However, FOREIGN KEY constraints are usually used for this purpose.
If constraints exist on the trigger table, they are checked after the INSTEAD OF trigger execution but prior to the AFTER trigger execution. If the constraints are violated, the INSTEAD OF trigger actions are rolled back and the AFTER trigger is not executed.