What are SQL Triggers?

-- MySQL
DELIMITER //
CREATE TRIGGER before_user_insert
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
  SET NEW.created_at = NOW();
  SET NEW.status = 'pending';
END //
DELIMITER ;

-- BEFORE INSERT
CREATE TRIGGER validate_email
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
  IF NEW.email NOT LIKE '%@%.%' THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Invalid email';
  END IF;
END;

-- AFTER INSERT
CREATE TRIGGER log_new_user
AFTER INSERT ON users
FOR EACH ROW
BEGIN
  INSERT INTO audit_log (action, table_name, record_id)
  VALUES ('INSERT', 'users', NEW.id);
END;

-- AFTER UPDATE
CREATE TRIGGER track_changes
AFTER UPDATE ON products
FOR EACH ROW
BEGIN
  IF OLD.price != NEW.price THEN
    INSERT INTO price_history (product_id, old_price, new_price)
    VALUES (NEW.id, OLD.price, NEW.price);
  END IF;
END;

-- AFTER DELETE
CREATE TRIGGER archive_deleted_user
AFTER DELETE ON users
FOR EACH ROW
BEGIN
  INSERT INTO deleted_users (id, name, email, deleted_at)
  VALUES (OLD.id, OLD.name, OLD.email, NOW());
END;

-- Show triggers
SHOW TRIGGERS;

-- Drop trigger
DROP TRIGGER before_user_insert;