DBMS: Week 12 - Triggers in MySQL Database Server

1. International Islamic University H-10, Islamabad, Pakistan
Database Managements Systems
Week 12
Triggers
Engr. Rashid Farid Chishti
http://youtube.com/rfchishti
http://sites.google.com/site/chisht
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2.  Understand the concept of triggers and their use cases.
 Learn how to create and manage triggers for automated actions in a database.
Learning Objectives

3.  Definition:
 A trigger is a set of SQL statements that automatically execute when a certain event
occurs on a table or view.
 Types of Triggers:
 Triggers in MySQL are classified based on timing and event type:
 Based on Timing
 BEFORE: Executes before an INSERT, UPDATE, or DELETE operation on a table.
 AFTER: Executes after an INSERT, UPDATE, or DELETE operation on a table.
 Based on Events
 INSERT: Fires when a new row is inserted into a table.
 UPDATE: Fires when an existing row is updated.
 DELETE: Fires when a row is deleted from a table.
What Are Triggers?

4.  Since triggers are defined based on timing and event type, MySQL supports
the following six trigger combinations:
 BEFORE INSERT: Fires before a new row is inserted.
 AFTER INSERT: Fires after a new row is inserted.
 BEFORE UPDATE: Fires before an existing row is updated.
 AFTER UPDATE: Fires after an existing row is updated.
 BEFORE DELETE: Fires before a row is deleted.
 AFTER DELETE: Fires after a row is deleted.
Trigger Combinations in MySQL

5.  Automatic Updates:
 Automatically update related records in other tables when data changes.
 Data Integrity:
 Enforce data integrity rules, such as checking for valid data or preventing
certain operations.
 Audit Logging:
 Track changes to important tables, such as logging every insert, update, or
delete operation.
 Prevent Invalid Operations:
 Prevent users from performing certain operations, like deleting a record that
is referenced by another table.
When to Use Triggers?

6.  Basic syntax to create a trigger:
CREATE TRIGGER trigger_name
BEFORE/AFTER INSERT/UPDATE/DELETE
ON table_name
FOR EACH ROW
BEGIN
-- SQL statements
END;
Syntax for Creating Triggers

7.  Example: First of all, create two tables
A Trigger Example
DROP DATABASE IF EXISTS week_12_db;
CREATE DATABASE week_12_db; USE week_12_db;
CREATE TABLE Orders (
id INT PRIMARY KEY AUTO_INCREMENT,
customer VARCHAR(100), product VARCHAR(100), quantity INT,
order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
CREATE TABLE Order_Logs (
log_id INT PRIMARY KEY AUTO_INCREMENT,
order_id INT,
action_type VARCHAR(50),
action_details VARCHAR(255),
logged_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

8.  Create an AFTER INSERT Trigger
Trigger Execution Example
DELIMITER $$
CREATE TRIGGER after_insert_order
AFTER INSERT ON Orders
FOR EACH ROW
BEGIN
-- Insert new order details into the order_logs table
INSERT INTO Order_Logs (order_id, action_type, action_details)
VALUES (
NEW.id,
'AFTER INSERT',
CONCAT('Order Inserted: ' , NEW.customer , ', ' ,
NEW.product , ', ' , NEW.quantity )
);
END $$
DELIMITER ;

9.  OLD and NEW
Trigger Execution Example
Context OLD NEW
INSERT Not available New row values
UPDATE Old values New values
DELETE Old values Not available

10.  Testing the AFTER INSERT Trigger
 View the Order_Logs Table
 SHOW TRIGGERS: Show existing triggers in a database.
 DROP TRIGGER: Delete an existing trigger.
Dropping Triggers
DROP TRIGGER after_insert_order;
INSERT INTO Orders (customer, product, quantity)
VALUES
('Alice Brown', 'Laptop' , 2 ),
('Bob White' , 'Smartphone', 1 );
SELECT * FROM Order_Logs;
SHOW TRIGGERS FROM week_12_db;

11.  Create an AFTER UPDATE Trigger
Creating AFTER UPDATE Trigger
DELIMITER $$
CREATE TRIGGER after_update_order
AFTER UPDATE ON Orders
FOR EACH ROW
BEGIN
-- Insert new order details into the order_logs table
INSERT INTO Order_Logs (order_id, action_type, action_details)
VALUES (
NEW.id,
'AFTER UPDATE',
CONCAT('Updated Customer from ' , OLD.customer , ' to ',
NEW.customer ,
' Product from, ' , OLD.product , ' to ', NEW.product
,
' Quantity from, ', OLD.quantity , ' to ',
NEW.quantity )
);

12.  Testing the AFTER INSERT Trigger
 View the Order_Logs Table
Testing AFTER UPDATE Trigger
UPDATE Orders SET customer = 'David', product = 'Desktop'
WHERE id = 2;
SELECT * FROM Order_Logs;
log_id order_id action_type action_details logged_at
1 1 AFTER INSERT Order Inserted: Alice Brown, Laptop, 2 4/4/2025 5:01
2 2 AFTER INSERT Order Inserted: Bob White, Smartphone, 1 4/4/2025 5:01
3 2 AFTER UPDATE
Updated Customer from Bob White to David
Product from, Smartphone to Desktop Quantity
from, 1 to 1 4/4/2025 5:01
Table: Order_Logs

13.  Create an AFTER DELETE Trigger to log deletions.
Creating AFTER DELETE Trigger
DELIMITER $$
CREATE TRIGGER after_delete_order
AFTER Delete ON Orders
FOR EACH ROW
BEGIN
-- Insert new order details into the order_logs table
INSERT INTO Order_Logs (order_id, action_type, action_details)
VALUES (
OLD.id,
'AFTER DELETE',
CONCAT('Deleted Customer ', OLD.customer ,
', Product ' , OLD.product ,
', Quantity ', OLD.quantity)
);
END $$
DELIMITER ;

14.  Testing the AFTER DELETE Trigger
 View the Order_Logs Table
Testing AFTER DELETE Trigger
DELETE FROM Orders WHERE id = 2;
SELECT * FROM Order_Logs;
log_id order_id action_type action_details logged_at
1 1 AFTER INSERT Order Inserted: Alice Brown, Laptop, 2 4/4/2025 5:01
2 2 AFTER INSERT Order Inserted: Bob White, Smartphone, 1 4/4/2025 5:01
3 2 AFTER UPDATE
Updated Customer from Bob White to David
Product from, Smartphone to Desktop Quantity
from, 1 to 1 4/4/2025 5:01
4 2 AFTER DELETE Deleted Customer David, Product Desktop,
Quantity 1 4/4/2025 5:02
Table: Order_Logs

15.  Performance Overhead:
 Triggers can slow down operations because they add additional processing.
 Complexity:
 If multiple triggers are used on the same table, their interactions can
become complex and difficult to manage.
 Debugging:
 Triggers can be harder to debug, especially when they execute automatically
based on events.
Limitations of Triggers

16.  Use triggers to enforce data integrity, but avoid overuse.
 Be careful with AFTER and BEFORE triggers as they can affect performance.
 Ensure triggers are optimized to avoid unnecessary processing.
Best Practices for using Triggers