1)对触发器，用其来实现数据完整性，举出两个业务规则例子，并使用触发器 加以实现。要求一个是 AFTER 类触发器，一个是 BEFORE 类触发器

业务规则1：订单表中的订单金额必须大于0，否则不允许插入数据。

AFTER 类触发器实现：

CREATE TRIGGER after_order_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  IF NEW.order_amount <= 0 THEN
    DELETE FROM orders WHERE order_id = NEW.order_id;
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = '订单金额必须大于0';
  END IF;
END;

BEFORE 类触发器实现：

CREATE TRIGGER before_order_insert
BEFORE INSERT ON orders
FOR EACH ROW
BEGIN
  IF NEW.order_amount <= 0 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = '订单金额必须大于0';
  END IF;
END;

业务规则2：员工表中的年龄必须大于等于18岁，否则不允许插入数据。

AFTER 类触发器实现：

CREATE TRIGGER after_employee_insert
AFTER INSERT ON employees
FOR EACH ROW
BEGIN
  IF YEAR(CURDATE()) - YEAR(NEW.birth_date) < 18 THEN
    DELETE FROM employees WHERE employee_id = NEW.employee_id;
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = '员工必须年满18岁';
  END IF;
END;

BEFORE 类触发器实现：

CREATE TRIGGER before_employee_insert
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
  IF YEAR(CURDATE()) - YEAR(NEW.birth_date) < 18 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = '员工必须年满18岁';
  END IF;
END;