MySQL 笔记8 触发器

参考：《MySQL必知必会》Ben Forta著，第25章使用触发器

触发器

考虑如下场景，
- 每当增加一个顾客到某个数据库表时，都检查其电话号码格式是否正确，州的缩写是否为大写；
- 每当订购一个产品时，都从库存数量中减去订购的数量；
- 无论何时删除一行，都在某个存档表中保留一个副本。
上述例子的共同之处 => 需要在某个表发生更改时自动处理 => 触发器

触发器是MySQL响应以下任意语句而自动执行的一条MySQL语句（或位于BEGIN和END语句之间的一组语句）：
DELETE
INSERT
UPDATE
其它MySQL语句不支持触发器。

创建触发器

创建触发器时，需要给出4条信息：
- 唯一的触发器名
- 触发器关联的表
- 触发器应该响应的活动（DELETE、INSERT或UPDATE）
- 触发器何时执行（处理之前或之后）
注意：
1.只有表才支持触发器，视图、临时表不支持。
2.触发器按每个表每个事件每次地定义 =>
每个表每个事件每次只允许一个触发器（每个表最多支持6个触发器）
单一触发器不能与多个事件或多个表关联

【例】每次往表products插入数据，执行SELECT 'Product added'

mysql> CREATE TRIGGER newproduct AFTER INSERT ON products
    -> FOR EACH ROW SELECT 'Product added';
ERROR 1415 (0A000): Not allowed to return a result set from a trigger
mysql>

报错原因：MYSQL5以后，不允许触发器返回任何结果
解决方法：使用 INTO @变量名，将结果赋值到变量中，再用SELECT调用

mysql> CREATE TRIGGER newproduct AFTER INSERT ON products FOR EACH ROW SELECT 'Product added' INTO @prompt;
Query OK, 0 rows affected (0.06 sec)

mysql> SELECT @prompt;
+---------+
| @prompt |
+---------+
| NULL    |
+---------+
1 row in set (0.00 sec)

mysql>

通过插入数据触发触发器，

mysql> DESC products;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| prod_id    | char(10)     | NO   | PRI | NULL    |       |
| vend_id    | int(11)      | NO   | MUL | NULL    |       |
| prod_name  | char(255)    | NO   |     | NULL    |       |
| prod_price | decimal(8,2) | NO   |     | NULL    |       |
| prod_desc  | text         | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

mysql> INSERT INTO products
    -> VALUES('SAFE2',1003,'Safe',50.00,'Safe with combination lock');
Query OK, 1 row affected (0.02 sec)

mysql> SELECT @prompt;
+---------------+
| @prompt       |
+---------------+
| Product added |
+---------------+
1 row in set (0.00 sec)

mysql> DELETE FROM products WHERE prod_id='SAFE2';
Query OK, 1 row affected (0.04 sec)

mysql>

删除触发器

DROP TRIGGER newproduct;
注意：触发器不能更新或覆盖。为了修改一个触发器，必须先删除它，然后再重新创建。

使用触发器

1. INSERT 触发器

- 在INSERT触发器代码内，可引用一个名为NEW的虚拟表，访问被插入的行；
- 在BEFORE INSERT触发器中，NEW中的值也可以被更新（允许更改被插入的值）；
- 通常，将BEFORE用于数据验证和净化（目的是保证插入表中的数据确实是需要的数据）,此项也适用于UPDATE触发器。

【例】插入一个新订单到orders表时，MySQL生成一个新订单号并保存到order_num中，触发器从NEW.order_num取得这个值
说明：此触发器必须按照AFTER INSERT执行，因为在BEFORE INSERT语句执行之前，新order_num还没有生成。

mysql> CREATE TRIGGER neworder AFTER INSERT ON orders
    -> FOR EACH ROW SELECT NEW.order_num INTO @insertmp;
Query OK, 0 rows affected (0.06 sec)

mysql> INSERT INTO orders(order_date,cust_id)
    -> VALUES(Now(),10001);
Query OK, 1 row affected (0.02 sec)

mysql> SELECT @insertmp;
+-----------+
| @insertmp |
+-----------+
|     20010 |
+-----------+
1 row in set (0.00 sec)

mysql> INSERT INTO orders(order_date,cust_id) VALUES(Now(),10001);
Query OK, 1 row affected (0.06 sec)

mysql> SELECT @insertmp;
+-----------+
| @insertmp |
+-----------+
|     20011 |
+-----------+
1 row in set (0.00 sec)

mysql>

2. DELETE 触发器

- 在DELETE触发器代码内，你可以引用一个名为OLD的虚拟表，访问被删除的行；
- OLD中的值全都是只读的，不能更新。
- 使用BEFORE DELETE触发器的优点是（相对于AFTER DELETE触发器），如果由于某种原因，订单不能存档，DELETE本身将被放弃。

【例】orders上创建DELETE触发器，将OLD中的值（要删除的订单）保存到存档表archive_orders

首先要新建存档表archive_orders，与表orders结构一样

mysql> SHOW CREATE TABLE orders\G;
*************************** 1. row ***************************
       Table: orders
Create Table: CREATE TABLE `orders` (
  `order_num` int(11) NOT NULL AUTO_INCREMENT,
  `order_date` datetime NOT NULL,
  `cust_id` int(11) NOT NULL,
  PRIMARY KEY (`order_num`),
  KEY `fk_orders_customers` (`cust_id`),
  CONSTRAINT `fk_orders_customers` FOREIGN KEY (`cust_id`) REFERENCES `customers` (`cust_id`)
) ENGINE=InnoDB AUTO_INCREMENT=20012 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

ERROR:
No query specified

mysql>
mysql> CREATE TABLE `archive_orders` (
    ->   `order_num` int(11) NOT NULL AUTO_INCREMENT,
    ->   `order_date` datetime NOT NULL,
    ->   `cust_id` int(11) NOT NULL,
    ->   PRIMARY KEY (`order_num`),
    ->   KEY `fk_orders_customers` (`cust_id`),
    ->   CONSTRAINT `fk_orders_customers` FOREIGN KEY (`cust_id`) REFERENCES `customers` (`cust_id`)
    -> ) ENGINE=InnoDB AUTO_INCREMENT=20012 DEFAULT CHARSET=latin1;
ERROR 1022 (23000): Can't write; duplicate key in table 'archive_orders'

外键冲突报错解决：换一个外键名，如下

mysql> CREATE TABLE `archive_orders` (
    ->   `order_num` int(11) NOT NULL AUTO_INCREMENT,
    ->   `order_date` datetime NOT NULL,
    ->   `cust_id` int(11) NOT NULL,
    ->   PRIMARY KEY (`order_num`),
    ->   KEY `fk_ordersbk_customers` (`cust_id`),
    ->   CONSTRAINT `fk_ordersbk_customers` FOREIGN KEY (`cust_id`) REFERENCES `customers` (`cust_id`)
    -> ) ENGINE=InnoDB AUTO_INCREMENT=20012 DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.22 sec)

mysql> select * from archive_orders;
Empty set (0.00 sec)

mysql>

然后，创建DELETE触发器

mysql> DELIMITER //
mysql> CREATE TRIGGER deleteorder BEFORE DELETE ON orders
    ->  FOR EACH ROW
    ->  BEGIN
    ->   INSERT INTO archive_orders(order_num,order_date,cust_id)
    ->   VALUES(OLD.order_num, OLD.order_date, OLD.cust_id);
    ->  END //
Query OK, 0 rows affected (0.05 sec)

mysql> DELIMITER ;

验证DELETE触发器

mysql> DELETE FROM orders WHERE order_num=20011;
Query OK, 1 row affected (0.02 sec)

mysql> select * from archive_orders;
+-----------+---------------------+---------+
| order_num | order_date          | cust_id |
+-----------+---------------------+---------+
|     20011 | 2020-03-29 15:15:56 |   10001 |
+-----------+---------------------+---------+
1 row in set (0.00 sec)

mysql>
mysql> DELETE FROM orders WHERE order_num=20010;
Query OK, 1 row affected (0.04 sec)

mysql> select * from archive_orders;
+-----------+---------------------+---------+
| order_num | order_date          | cust_id |
+-----------+---------------------+---------+
|     20010 | 2020-03-29 15:15:18 |   10001 |
|     20011 | 2020-03-29 15:15:56 |   10001 |
+-----------+---------------------+---------+
2 rows in set (0.00 sec)

mysql>

3. UPDATE 触发器

- 在UPDATE触发器代码中，引用虚拟表OLD访问UPDATE语句前的值，引用虚拟表NEW访问更新的值；
- 在BEFORE UPDATE触发器中，NEW中的值可能也被更新（允许更改将要用于UPDATE语句中的值）；
- OLD中的值全都是只读的，不能更新。

【例】保证州名缩写总是大写（不管UPDATE语句中给出的是大写还是小写）

mysql> CREATE TRIGGER updatevendor BEFORE UPDATE ON vendors
    -> FOR EACH ROW SET NEW.vend_state = Upper(New.vend_state);
Query OK, 0 rows affected (0.05 sec)

mysql> UPDATE vendors SET vend_state='ca' WHERE vend_id=1004;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM vendors WHERE vend_id=1004;;
+---------+--------------+-----------------+-----------+------------+----------+--------------+
| vend_id | vend_name    | vend_address    | vend_city | vend_state | vend_zip | vend_country |
+---------+--------------+-----------------+-----------+------------+----------+--------------+
|    1004 | Furball Inc. | 1000 5th Avenue | New York  | CA         | 11111    | USA          |
+---------+--------------+-----------------+-----------+------------+----------+--------------+
1 row in set (0.00 sec)

ERROR:
No query specified

mysql> UPDATE vendors SET vend_state='ny' WHERE vend_id=1004;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM vendors WHERE vend_id=1004;;
+---------+--------------+-----------------+-----------+------------+----------+--------------+
| vend_id | vend_name    | vend_address    | vend_city | vend_state | vend_zip | vend_country |
+---------+--------------+-----------------+-----------+------------+----------+--------------+
|    1004 | Furball Inc. | 1000 5th Avenue | New York  | NY         | 11111    | USA          |
+---------+--------------+-----------------+-----------+------------+----------+--------------+
1 row in set (0.00 sec)

ERROR:
No query specified

mysql>

MySQL 笔记8 触发器

触发器

创建触发器

删除触发器

使用触发器

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