MySQL触发器的使用

概述：

  当具体的表发生特定的数据库事件时，触发器执行对应的SQL命令。

语法：

创建触发器的一般命令如下：

CREATE [temp|temporary] trigger name

[before|after] [insert|delete|update|update of columns] ON table

action

触发器的使用：

创建：

mysql> CREATE TRIGGER stu_trigger AFTER INSERT
    -> ON students
    -> FOR EACH ROW
    -> INSERT INTO info(stu_id, info) values (new.id, '');
Query OK, 0 rows affected (0.07 sec)

触发事件：

mysql> INSERT INTO students(id, name, age) values (4, 'Zeus', 56400);
Query OK, 1 row affected (0.00 sec)

验证students表结果：

mysql> SELECT * FROM students;
+------+----------------+--------+
| id   | name           | age    |
+------+----------------+--------+
|    1 | bumblebee      |    800 |
|    2 | king of monkey |  10000 |
|    3 | Medusa         | 100000 |
|    4 | Zeus           |  56400 |
+------+----------------+--------+
4 rows in set (0.00 sec)

验证info表结果：

mysql> SELECT * FROM info;
+----+--------+---------------------------------+
| id | stu_id | info                            |
+----+--------+---------------------------------+
|  1 |      1 | A member of the deformed steel. |
|  2 |      2 | Hero in Chinese Mythology.      |
|  3 |      3 | In Greek mythology the Gorgon.  |
|  5 |      4 |                                 |
+----+--------+---------------------------------+
4 rows in set (0.00 sec)

注：这里的新id为5而不为4的原因，是因为之前对info表作了删除操作导致。

查看触发器的信息：

mysql> SELECT * FROM information_schema.triggers;
+----------------+--------------+--------------------+---------------------+
| TRIGGER_SCHEMA | TRIGGER_NAME | EVENT_MANIPULATION | EVENT_OBJECT_SCHEMA |
+----------------+--------------+--------------------+---------------------+
| student        | stu_trigger  | INSERT             | student             |
+----------------+--------------+--------------------+---------------------+


+--------------------+----------------------------------------------------+
| EVENT_OBJECT_TABLE | ACTION_STATEMENT                                   |
+--------------------+----------------------------------------------------+
| students           | INSERT INTO info(stu_id, info) values (new.id, '') |
+--------------------+----------------------------------------------------+

1 row in set (0.19 sec)

注：上面的打印信息是删减版的，完整的信息，可以自行在环境中查看。

删除触发器：

mysql> DROP TRIGGER stu_trigger;
Query OK, 0 rows affected (0.01 sec)

验证删除结果：

mysql> SELECT * FROM information_schema.triggers;
Empty set (0.01 sec)

说明删除成功了