1. Triggers Event handlers in the DBMS

2. Triggers are event handlers Triggers a executed when an event happens in the DBMS Example events – INSERT, UPDATE and DELETE Triggers contain [a little] code written in T-SQL The code is executed before or after the INSERT, UPDATE or DELETE – Or sometimes INSTEAD OF the INSERT, UPDATE or DELETE! 2Triggers

3. Syntax Create syntax CREATE TRIGGER someName ON someTableName | viewName FOR | AFTER | INSTEAD OF DELETE | INSERT | UPDATE AS T-SQL statements 3Triggers

4. Transparency The database user does not know about triggers There might be more than one trigger taking care of a single event on a certain table – It can be quite advanced and confusing to debug! 4Triggers

5. Triggers enforce business rules If a constraint on a single table is not enough, trigger might be handy. – Triggers can do many thing, to many tables 5Triggers

6. Example: Name cannot change CREATE TRIGGER trStudentCannotChangeName ON student FOR UPDATE AS BEGIN IF UPDATE(name) BEGIN raiserror('Can not change student name', 16, 1); rollback tran; END Triggers6

7. Example: Teachers cannot be deleted Teachers will always be teachers CREATE TRIGGER trNoTeacherDelete ON teacher FOR DELETE AS BEGIN raiserror('You cannot delete a teacher', 16, 1); END; Triggers7

8. Example: Teachers salary cannot go down CREATE TRIGGER trTeacherCannotLowerSalary ON teacher FOR UPDATE AS IF EXISTS ( SELECT 'true' FROM inserted JOIN deleted ON inserted. ID = deleted.ID WHERE deleted.salary > inserted.salary ) BEGIN RAISERROR('Cannot lower salary', 16, 1) ROLLBACK TRAN END In an FOR UPDATE trigger you have access to the old (deleted) data and the new (inserted) data 8Triggers

9. Example: Cannot raise teachers salary more than 50 percent CREATE TRIGGER teacherSalaryChange ON teacher for UPDATE AS BEGIN IF EXISTS (SELECT 'true' FROM inserted JOIN deleted ON inserted.ID = deleted.ID WHERE (inserted.salary - deleted.salary)/deleted.salary > 0.5) BEGIN raiserror('Cannot raise salary more than 50 percent', 16, 1); rollback tran; END Triggers9

10. INSTEAD OF triggers INSTEAD OF triggers is executed instead of the real action – Not before, or after Useful to update views – Many views are not updateable Example view CREATE VIEW vTeacherDepartment AS SELECT ID, Name, salary, departmentName FROM teacher INNER JOIN department ON teacher.departmentID = department.departmentID; Example INSERT insert into vTeacherDepartment (teacherName, salary, departmentname) values ('Anders', 444, 'Computer Science'); Result – Msg 4405, Level 16, State 1, Line 1 – View or function 'vTeacherDepartment' is not updatable because the modification affects multiple base tables. 10Triggers

11. INSTEAD OF triggers INSERT on a join view: Throwing exceptions CREATE TRIGGER trTeacherDepartmentInsert ON vTeacherDepartment INSTEAD OF INSERT AS BEGIN DECLARE @departmentID INT; SET @departmentID = (SELECT departmentID FROM department JOIN inserted ON department.departmentName = inserted.departmentName); IF (@departmentID IS NULL) BEGIN DECLARE @errormessage varchar(50); SET @errormessage = 'No such department: ' + (SELECT departmentName FROM inserted); RAISERROR(@errormessage, 16, 1); ROLLBACK TRAN; END ELSE INSERT INTO teacher (name, salary, departmentID) SELECT name, salary, @departmentID FROM inserted; END 11Triggers

12. INSTEAD OF triggers INSERT on a join view: Double insert ALTER TRIGGER trTeacherDepartmentInsert ON vTeacherDepartment INSTEAD OF INSERT AS BEGIN DECLARE @departmentID INT; SET @departmentID = (SELECT departmentID FROM department JOIN inserted ON department.departmentName = inserted.departmentName); IF (@departmentID IS NULL) BEGIN DECLARE @deparmentname VARCHAR(100); INSERT INTO department (departmentName) SELECT departmentname FROM inserted SET @departmentID = @@IDENTITY; -- last identity value assigned in the current connection END INSERT INTO teacher (name, salary, departmentID) SELECT name, salary, @departmentID FROM inserted; END 12Triggers

13. Finding existing triggers using Microsoft SQL Server Management Studio Triggers13