Triggers are stored procedures that run automatically in response to data changes like inserts, updates, and deletes. There are two types of triggers: DDL triggers respond to data definition language statements like CREATE and ALTER, while DML triggers respond to data manipulation language statements that change data. DML triggers can be FOR or AFTER triggers that fire after the data change, or INSTEAD OF triggers that fire before and allow modifying the change. Triggers enable enforcing business rules and logging actions across tables in ways that constraints cannot.

1. CIS-282

2. Triggers
 Specialized stored procedure that runs ‘automatically’
 Another method to enforce rules
 Typically used to enforce business rules not easily done
by constraint
 Often used when need to log actions

3. DDL v. DML Triggers
 DDL Triggers allow procedures to run as part of
CREATE, ALTER, and DROP statements
 DML Triggers run in response to data changes –
INSERT, UPDATE, DELETE

4. DML Trigger Types
 For/After: Fire after the action has occurred (tables
only)
Changes written to data tables Trigger fires
 Instead Of: Fire after changes written to logical tables
but before written to data tables (tables or views)
Trigger fires Changes written to data tables

5. Respond to actions on data
 Trigger is not called
 Trigger is specific to a table (or view)
 Table can have multiple triggers
 One trigger can respond to different actions

6. Triggers & Constraints
 Constraints give better performance
 Triggers can span tables, databases, servers;
constraints are table specific
 If testing a value inserted in one table depends on an
entry in another table, use a trigger
 If testing a value depends on an entry in the current row,
a relationship, or function result, use a constraint

7. Syntax
CREATE TRIGGER <trigger name>
ON <table name>
FOR <insert, update, delete>
AS
…

8. Related Tables
 Triggers can use the Inserted and Deleted tables
 Inserted table holds new or updated records
 Deleted table holds deleted or original version of
updated records

9. Inserted & Deleted Tables
Action Inserted Table Deleted Table
Insert XX
Update XX XX
Delete XX

10. Triggers for Referential Integrity
 If trying to enroll a student, add a trigger on
insert or update that tests for valid student id
in Students table
If Exists (Select * from Students
Where StudentID IN
(Select StudentID From Inserted))
Begin
Insert into Enrollments(…)
End

11. Update()
 Can test whether a column has changed
 Can use to determine if a calculated field need to be
updated
 Update(columnname)
If Update(FirstName)
Print ‘FirstName has changed’

12. Updating Summary Information
 If tracking Possible Points for a schedule, changing
Possible Points for an Assignment would require
change to the Schedule’s Possible Points
 Use Update(PossiblePoints) in trigger for Assignment
insert, update

13. Instead Of Triggers
 Can have one for each type of action
 Can apply to views
 Provides way to allow data modification in joined tables
 Instead of triggers won’t work if have cascade update
or delete
 Need to complete action!
 If you’re executing code instead of deleting a row, the
row won’t be deleted if you don’t delete it in your code!