【mysql】mysql触发器before与after的区别

1.商品表

-- 创建商品表
DROp TABLE IF EXISTS `goods`;
CREATE TABLE `goods`  (
  `id` int(0) NOT NULL AUTO_INCREMENT COMMENT '商品id',
  `name` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '商品名称',
  `num` int(0) DEFAULT NULL COMMENT '数量',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of goods
-- ----------------------------
INSERT INTO `goods` VALUES (1, '辣条', -10);
INSERT INTO `goods` VALUES (2, '棒棒糖', 10);
INSERT INTO `goods` VALUES (3, '酸奶', 10);

2.订单表

-- 创建订单表
DROP TABLE IF EXISTS `order`;
CREATE TABLE `order`  (
  `id` int(0) NOT NULL AUTO_INCREMENT COMMENT '订单id',
  `goods_id` int(0) DEFAULT NULL COMMENT '商品id',
  `num` int(0) DEFAULT NULL COMMENT '商品数量',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 1 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of order
-- ----------------------------
INSERT INTO `order` VALUES (1, 1, 20);

3.触发器

-- 创建触发器trigger1
drop trigger if exists trigger1;
DELIMITER $$
create trigger trigger1 after insert on `order` for each row 
begin
 update goods set num=num-new.num where id = new.goods_id;
end $$

//查看结果
select * from `order`;
insert into `order`(goods_id,num) values(1,20);
select * from `goods`;

//创建触发器
drop trigger if exists trigger2;
DELIMITER $$
create trigger trigger2 before insert on `order` for each row 
begin
 if new.num > 10 then
  set new.num = 10;
 end if;
 update goods set num=num-new.num where id = new.goods_id;
end $$

//查看结果
select * from `order`;
insert into `order`(goods_id,num) values(1,20);
select * from `goods`;

//mysql触发器中的before和after的区别：

before：(insert、update)可以对new进行修改，
after:不能对new进行修改，两者都不能修改old数据。

insert语句只有new是合法的,新增的数据没有old
delete语句只有old是合法的,删除的数据没有new
update语句new、old可以同时使用,有old new 两份

after是先完成数据的增删改，再触发，触发的语句晚于监视的增删改操作，无法影响前面的增删改动作；也就是说先插入订单记录，再更新商品的数量；

before是先完成触发，再增删改，触发的语句先于监视的增删改，这样就可以对new进行修改