🔥 Now, let’s move to the next topic:

Triggers in SQL
(Automation inside database 💯)

🧠 1. What is a Trigger?
A Trigger is a special SQL block
👉 that runs automatically
👉 when an event happens in a table

Think like this 👇
👉 “Automatic action on INSERT / UPDATE / DELETE”

⚡ 2. Why Use Triggers?
✔ Automatic logging
✔ Data validation
✔ Audit tracking
✔ Prevent invalid operations

⚡ 3. Types of Triggers
BEFORE INSERT → Runs before inserting data
AFTER INSERT → Runs after inserting data
BEFORE UPDATE → Runs before updating
AFTER UPDATE → Runs after updating
BEFORE DELETE → Runs before deleting
AFTER DELETE → Runs after deleting

🔥 4. Basic Trigger Example
👉 Automatically log inserted employee

CREATE TABLE employee_log (
log_message VARCHAR(255)
);

DELIMITER //

CREATE TRIGGER after_employee_insert
AFTER INSERT ON employees
FOR EACH ROW
BEGIN
INSERT INTO employee_log
VALUES (CONCAT('New employee added: ', NEW.name));
END //

DELIMITER ;

🧠 5. Important Keywords
NEW → New inserted/updated value
OLD → Previous value before update/delete

⚡ 6. BEFORE UPDATE Example
👉 Prevent negative salary
DELIMITER //

CREATE TRIGGER check_salary
BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
IF NEW.salary < 0 THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Salary cannot be negative';
END IF;
END //

DELIMITER ;

❌ 7. Drop Trigger
DROP TRIGGER after_employee_insert;

🎯 8. Practice Tasks
1. Create AFTER INSERT trigger
2. Create BEFORE UPDATE trigger
3. Prevent negative salary using trigger
4. Log deleted employees
5. Drop created trigger

⚡ Mini Challenge 🔥
👉 Create trigger to automatically save deleted employee names into another table

🔥 Mini Challenge Solution

👉 Automatically save deleted employee names into another table

✅ Step 1: Create Log Table

CREATE TABLE deleted_employees (
emp_id INT,
name VARCHAR(50),
deleted_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

✅ Step 2: Create Trigger

DELIMITER //

CREATE TRIGGER log_deleted_employee
AFTER DELETE ON employees
FOR EACH ROW
BEGIN
INSERT INTO deleted_employees(emp_id, name)
VALUES (OLD.emp_id, OLD.name);
END //

DELIMITER ;

🧠 How It Works

👉 AFTER DELETE → runs automatically after deletion

👉 OLD.emp_id and OLD.name
Access deleted row values before they disappear

✅ Example

DELETE FROM employees
WHERE emp_id = 101;

✔ Deleted employee info automatically saved in deleted_employees table 💯

🔥 Pro Tip
Triggers are powerful but:
❌ Too many triggers can slow database
✅ Use them carefully 💯

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