需求：我有一张心愿表，还有一张捐款表，捐款表每次支付或修改，相应的心愿表总金额也会随之修改。

心愿表：

CREATE TABLE `qs_wish`  (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
  `title` varchar(255) CHARACTER '心愿标题',
  `total_amount` decimal(10, 2) DEFAULT 0.00 COMMENT '捐赠总金额',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

捐款表：

CREATE TABLE `qs_wish_donate`  (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
  `wish_id` bigint(20) NOT NULL DEFAULT 0 COMMENT '心愿id',
  `amount` decimal(10, 2) NOT NULL DEFAULT 0.00 COMMENT '捐款',
  `status` tinyint(4) NOT NULL DEFAULT 0 COMMENT '支付状态',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 2 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

触发器代码

新增：新增时只要状态（status）为1，心愿表总金额也随之增加。

CREATE TRIGGER `tri_wish_donate_insert_after` AFTER INSERT ON `qs_wish_donate` FOR EACH ROW begin
if new.status = 1 then 
    UPDATE qs_wish SET total_amount=total_amount+new.amount WHERE id=new.wish_id;
END if;
end

修改：修改可大致分为三种情况

1、将状态由0改成1，即由待支付状态改成支付状态。

2、将状态由1改成0或其他值，即由支付状态改成待支付或退回状态。

3、状态未修改，且状态为1，但是修改了心愿id或者金额，异或两者都被修改。

三种情况代码如下：

CREATE TRIGGER `tri_wish_donate_update_after` AFTER UPDATE ON `qs_wish_donate` FOR EACH ROW begin
if new.status =1 and old.status != 1 then 
    UPDATE qs_wish SET total_amount=total_amount+new.amount WHERE id=new.wish_id;
elseif old.status = 1 and  new.status != 1 then 
    UPDATE qs_wish SET total_amount=total_amount-old.amount WHERE id=old.wish_id;
elseif old.status=1 and new.status=1 and (old.wish_id != new.wish_id or old.amount != new.amount) then
    UPDATE qs_wish SET total_amount=total_amount-old.amount WHERE id=old.wish_id;
    UPDATE qs_wish SET total_amount=total_amount+new.amount WHERE id=new.wish_id;
END if;
end

删除：删除只关注已支付的捐款，即status=1的情况

CREATE TRIGGER `tri_wish_donate_delete_after` AFTER DELETE ON `qs_wish_donate` FOR EACH ROW begin
if old.status = 1 then 
    UPDATE qs_wish SET total_amount=total_amount-old.amount WHERE id=old.wish_id;
END if;
end