MySQL-触发器

1、什么叫触发器：
	当满足一定的条件以后，它会触发一个动作的执行，trigger
2、触发器的执行不是由程序调用，也不是由手工启动，而是由事件来触发、**从而实现执行
3、触发器是由一定的事件来触发的，对表的增删改操作、不包括查询，查询是没有触发器的
4、生产中一般不通过MySQL中触发器来实现这功能，是通过Java程序、Python程序代码来实现触发器
5、创建触发器：
		CREATE：
			[DEFINER = { user | CURRENT_USER }]
			TRIGGER trigger_name
			trigger_time trigger_event
			ON tbl_name FOR EACH ROW
			trigger_body
		说明：
			trigger_name：触发器的名称
			trigger_time：{ BEFORE | AFTER }，表示在事件之前或之后触发
			trigger_event:：{ INSERT |UPDATE | DELETE }，触发的具体事件
			tbl_name：该触发器作用在表名
6、查看触发器
		SHOW TRIGGERS
		查询系统表information_schema.triggers的方式指定查询条件，查看指定的触发器信息。
		mysql> USE information_schema;
		Database changed
		mysql> SELECT * FROM triggers WHERE
7、删除触发器
		DROP TRIGGER trigger_name;
		快速的清空表：truncate table student_info

触发器示例

	创建触发器，在向学生表INSERT数据时，学生数增加，DELETE学生时，学生数减少
	MariaDB [hellodb]> use db1
	Database changed
	MariaDB [db1]> CREATE TABLE student_info (
	    -> stu_id INT(11) NOT NULL AUTO_INCREMENT,
	    -> stu_name VARCHAR(255) DEFAULT NULL,
	    -> PRIMARY KEY (stu_id)
	    -> );
	CREATE TABLE student_count (
	student_count INT(11) DEFAULT 0
	);
	INSERT INTO student_count VALUES(0);Query OK, 0 rows affected (0.11 sec)

	MariaDB [db1]> CREATE TABLE student_count (
	    -> student_count INT(11) DEFAULT 0
	    -> );
	Query OK, 0 rows affected (0.01 sec)

	MariaDB [db1]> INSERT INTO student_count VALUES(0);
	Query OK, 1 row affected (0.01 sec)

	MariaDB [db1]> select * from student_info;
	Empty set (0.00 sec)

	创建增加和减少学生数量的触发器
	MariaDB [db1]> CREATE TRIGGER trigger_student_count_insert
	    -> AFTER INSERT
	    -> ON student_info FOR EACH ROW
	    -> UPDATE student_count SET student_count=student_count+1;
	Query OK, 0 rows affected (0.00 sec)

	MariaDB [db1]> CREATE TRIGGER trigger_student_count_delete
	    -> AFTER DELETE
	    -> ON student_info FOR EACH ROW
	    -> UPDATE student_count SET student_count=student_count-1;
	Query OK, 0 rows affected (0.00 sec)

	# 查看创建好的触发器：
	MariaDB [db1]> show triggers\G
	*************************** 1. row ***************************
	             Trigger: trigger_student_count_insert
	               Event: INSERT
	               Table: student_info
	           Statement: UPDATE student_count SET student_count=student_count+1
	              Timing: AFTER
	             Created: 2018-10-09 20:27:09.78
	            sql_mode: STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
	             Definer: [email protected]
	character_set_client: utf8
	collation_connection: utf8_general_ci
	  Database Collation: utf8_general_ci
	*************************** 2. row ***************************
	             Trigger: trigger_student_count_delete
	               Event: DELETE
	               Table: student_info
	           Statement: UPDATE student_count SET student_count=student_count-1
	              Timing: AFTER
	             Created: 2018-10-09 20:27:27.31
	            sql_mode: STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
	             Definer: [email protected]
	character_set_client: utf8
	collation_connection: utf8_general_ci
	  Database Collation: utf8_general_ci
	2 rows in set (0.00 sec)

	查看建好的student_info表，目前是空的
	MariaDB [db1]> select * from student_info;
	Empty set (0.00 sec)

	往里面插入一条数据；
	MariaDB [db1]> insert student_info values(1,'cobbler');
	Query OK, 1 row affected (0.01 sec)

	查看student_info表里面已经有了一条信息
	MariaDB [db1]> select * from student_info;
	+--------+----------+
	| stu_id | stu_name |
	+--------+----------+
	|      1 | cobbler  |
	+--------+----------+
	1 row in set (0.00 sec)
	查看student_count里面的统计数，
	MariaDB [db1]> select * from student_count;
	+---------------+
	| student_count |
	+---------------+
	|             1 |
	+---------------+
	1 row in set (0.00 sec)