mysql创建触发器报错 mysql创建触发器例子

我们可以可以使用CREATE TRIGGER语句创建一个新的触发器，来看下具体的语法：

CREATE TRIGGER trigger_name trigger_time trigger_event
 ON table_name
 FOR EACH ROW
 BEGIN
 ...
 END;

CREATE TRIGGER trigger_name trigger_time trigger_event
 ON table_name
 FOR EACH ROW
 BEGIN
 ...
 END;

然后我们来详细看下上述sql的具体含义：

• 将触发器名称放在CREATE TRIGGER语句之后。触发器名称应遵循命名约定[trigger time]_[table name]_[trigger event]，例如before_employees_update。
• 触发激活时间可以在之前或之后。必须指定定义触发器的激活时间。如果要在更改之前处理操作，则使用BEFORE关键字，如果在更改后需要处理操作，则使用AFTER关键字。
• 触发事件可以是INSERT，UPDATE或DELETE。此事件导致触发器被调用。 触发器只能由一个事件调用。要定义由多个事件调用的触发器，必须定义多个触发器，每个事件一个触发器。
• 触发器必须与特定表关联。没有表触发器将不存在，所以必须在ON关键字之后指定表名。
• 将SQL语句放在BEGIN和END块之间。这是定义触发器逻辑的位置。

大概了解了之后，我们就来尝试创建触发器来记录employees表中行数据的更改情况，先来看下这个表的结构：

mysql> DESC employees;
+----------------+--------------+------+-----+---------+-------+
| Field          | Type         | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| employeeNumber | int(11)      | NO   | PRI | NULL    |       |
| lastName       | varchar(50)  | NO   |     | NULL    |       |
| firstName      | varchar(50)  | NO   |     | NULL    |       |
| extension      | varchar(10)  | NO   |     | NULL    |       |
| email          | varchar(100) | NO   |     | NULL    |       |
| officeCode     | varchar(10)  | NO   | MUL | NULL    |       |
| reportsTo      | int(11)      | YES  | MUL | NULL    |       |
| jobTitle       | varchar(50)  | NO   |     | NULL    |       |
+----------------+--------------+------+-----+---------+-------+
8 rows in set
mysql> DESC employees;
+----------------+--------------+------+-----+---------+-------+
| Field          | Type         | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| employeeNumber | int(11)      | NO   | PRI | NULL    |       |
| lastName       | varchar(50)  | NO   |     | NULL    |       |
| firstName      | varchar(50)  | NO   |     | NULL    |       |
| extension      | varchar(10)  | NO   |     | NULL    |       |
| email          | varchar(100) | NO   |     | NULL    |       |
| officeCode     | varchar(10)  | NO   | MUL | NULL    |       |
| reportsTo      | int(11)      | YES  | MUL | NULL    |       |
| jobTitle       | varchar(50)  | NO   |     | NULL    |       |
+----------------+--------------+------+-----+---------+-------+
8 rows in set

我们再来创建一个名为employees audit的新表，用来保存employees表中数据的更改：

CREATE TABLE employees_audit (
    id INT AUTO_INCREMENT PRIMARY KEY,
    employeeNumber INT NOT NULL,
    lastname VARCHAR(50) NOT NULL,
    changedat DATETIME DEFAULT NULL,
    action VARCHAR(50) DEFAULT NULL
);
CREATE TABLE employees_audit (
    id INT AUTO_INCREMENT PRIMARY KEY,
    employeeNumber INT NOT NULL,
    lastname VARCHAR(50) NOT NULL,
    changedat DATETIME DEFAULT NULL,
    action VARCHAR(50) DEFAULT NULL
);

再来创建一个BEFORE UPDATE触发器，该触发器在对employees表中的行记录更改之前被调用：

DELIMITER $$
CREATE TRIGGER before_employee_update 
    BEFORE UPDATE ON employees
    FOR EACH ROW 
BEGIN
    INSERT INTO employees_audit
    SET action = 'update',
     employeeNumber = OLD.employeeNumber,
        lastname = OLD.lastname,
        changedat = NOW(); 
END$$
DELIMITER ;

DELIMITER $$
CREATE TRIGGER before_employee_update 
    BEFORE UPDATE ON employees
    FOR EACH ROW 
BEGIN
    INSERT INTO employees_audit
    SET action = 'update',
     employeeNumber = OLD.employeeNumber,
        lastname = OLD.lastname,
        changedat = NOW(); 
END$$
DELIMITER ;

在上述触发器的主体中，我们使用OLD关键字来访问受触发器影响的行的employeeNumber和lastname列。我们要注意的是，在为insert定义的触发器中，可以仅使用NEW关键字。不能使用OLD关键字。但是，在为DELETE定义的触发器中，没有新行，因此您只能使用OLD关键字。在update触发器中，OLD是指更新前的行，而NEW是更新后的行。

然后，我们可以使用SHOW TRIGGERS语句，来查看数据库中的触发器：

mysql> SHOW TRIGGERS;
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
| Trigger                | Event  | Table     | Statement                                                                                                                                                             | Timing | Created                | sql_mode                                                                          | Definer        | character_set_client | collation_connection | Database Collation |
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
| before_employee_update | UPDATE | employees | BEGIN
    INSERT INTO employees_audit
    SET action = 'update',
     employeeNumber = OLD.employeeNumber,
        lastname = OLD.lastname,
        changedat = NOW();
END | BEFORE | 2017-08-02 22:06:36.40 | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | root@localhost | utf8                 | utf8_general_ci      | utf8_general_ci    |
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
1 row in set
mysql> SHOW TRIGGERS;
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
| Trigger                | Event  | Table     | Statement                                                                                                                                                             | Timing | Created                | sql_mode                                                                          | Definer        | character_set_client | collation_connection | Database Collation |
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
| before_employee_update | UPDATE | employees | BEGIN
    INSERT INTO employees_audit
    SET action = 'update',
     employeeNumber = OLD.employeeNumber,
        lastname = OLD.lastname,
        changedat = NOW();
END | BEFORE | 2017-08-02 22:06:36.40 | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | root@localhost | utf8                 | utf8_general_ci      | utf8_general_ci    |
+------------------------+--------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+------------------------+-----------------------------------------------------------------------------------+----------------+----------------------+----------------------+--------------------+
1 row in set

完事我们就来更新employees表以检查触发器是否被调用：

UPDATE employees 
SET 
    lastName = 'Maxsu'
WHERE
    employeeNumber = 1056;

UPDATE employees 
SET 
    lastName = 'Maxsu'
WHERE
    employeeNumber = 1056;

可以使用以下查询来查询employees_audit表，用以检查触发器是否被UPDATE语句调用：

mysql> SELECT * FROM employees_audit;
+----+----------------+----------+---------------------+--------+
| id | employeeNumber | lastname | changedat           | action |
+----+----------------+----------+---------------------+--------+
|  1 |           1056 | Hill     | 2017-08-02 22:15:51 | update |
+----+----------------+----------+---------------------+--------+
1 row in set
mysql> SELECT * FROM employees_audit;
+----+----------------+----------+---------------------+--------+
| id | employeeNumber | lastname | changedat           | action |
+----+----------------+----------+---------------------+--------+
|  1 |           1056 | Hill     | 2017-08-02 22:15:51 | update |
+----+----------------+----------+---------------------+--------+
1 row in set

如上面输出结果所示，触发器被真正调用，并在employees_audit表中插入一个新行。

好啦，本次记录就到这里了。

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