06.数据库触发器
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
触发器是⼀种特殊类型的存储过程，它是在特定的表上⾃动执⾏的代码。当特定的事件发⽣时，MySQL 触发器会
被⾃动激活。这些事件可能包括 INSERT、UPDATE 或 DELETE 操作，以及对特定列的更改。
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、激活从⽽实现执⾏
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:
1. DEFINER：指定触发器的创建者，可以是⽤户或当前⽤户。
2. TRIGGER trigger_name：定义触发器的名称。
3. trigger_time：指定触发器的触发时间，可以是BEFORE或AFTER。
4. trigger_event：指定触发器的触发事件，可以是INSERT、UPDATE或DELETE。
5. ON tbl_name：指定触发器所在的表名。
6. FOR EACH ROW：表示触发器对每⼀⾏记录都会触发。
7. trigger_body：触发器的执⾏语句体，可以是SQL语句或存储过程。
Demo需求: 在向⽤户表中插⼊新记录时，⾃动将该操作记录到⽤户审计表中：
1. 创建⼀个⽤户表和⼀个⽤户审计表：
CREATE TABLE users (
id INT(11) AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(50),
email VARCHAR(50)
);
CREATE TABLE user_audit (
id INT AUTO_INCREMENT PRIMARY KEY,
user_id INT,
action VARCHAR(50),
timestamp TIMESTAMP
);
2. 创建⼀个触发器，以便在向⽤户表中插⼊新记录时，⾃动将该操作记录到⽤户审计表中：
DELIMITER //
CREATE TRIGGER insert_user_audit
AFTER INSERT ON users
FOR EACH ROW
BEGIN
INSERT INTO user_audit (user_id, action, timestamp)
VALUES (NEW.id, 'INSERT', NOW());
END//
DELIMITER ;
具体解释如下：
CREATE TRIGGER：创建触发器的关键字。
user_trigger：触发器的名称。
AFTER INSERT ON user：触发器的触发时间和触发事件，表示在向⽤户表中插⼊新记录之后触发该触发器。
FOR EACH ROW：表示触发器对每⼀⾏记录都会触发。
BEGIN 和 END：表示触发器的执⾏体开始和结束。
INSERT INTO user_audit：将操作记录插⼊到⽤户审计表中。
(user_id, action, date)：指定插⼊的列。
VALUES (NEW.user_id, 'insert', NOW())：指定插⼊的值，其中NEW.user_id表示新插⼊的⽤户ID，'insert'表
示操作类型为插⼊，NOW()表示当前时间。
该触发器会在每次向⽤户表中插⼊新记录时，⾃动将该操作记录到⽤户审计表中。
3. 向⽤户表中插⼊⼀条新记录，例如：
INSERT INTO users (id, name, email) VALUES (1, 'Wing', 'Wing@qq.com');
4. 触发器就会⾃动将该操作记录到⽤户审计表中。通过以下查询来验证这⼀点：
SELECT * FROM user_audit
显示⼀个包含刚才插⼊的记录的结果集，其中包含⽤户 ID、动作类型（在本例中为“INSERT”）和时间戳
5、查看触发器
mysql> SHOW TRIGGERS\G
*************************** 1. row ***************************
Trigger: insert_user_audit
Event: INSERT
Table: users
Statement: BEGIN
INSERT INTO user_audit (user_id, action, timestamp)
VALUES (NEW.id, 'INSERT', NOW());
END
Timing: AFTER
Created: 2023-05-25 11:18:37.63
sql_mode:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_
BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
Definer: root@localhost
character_set_client: utf8
collation_connection: utf8_general_ci
Database Collation: latin1_swedish_ci
1 row in set (0.00 sec)
#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 information_schema.triggers WHERE trigger_schema = 'wing' AND
trigger_name = 'insert_user_audit'\G
*************************** 1. row ***************************
TRIGGER_CATALOG: def
TRIGGER_SCHEMA: wing
TRIGGER_NAME: insert_user_audit
EVENT_MANIPULATION: INSERT
EVENT_OBJECT_CATALOG: def
EVENT_OBJECT_SCHEMA: wing
EVENT_OBJECT_TABLE: users
ACTION_ORDER: 1
ACTION_CONDITION: NULL
ACTION_STATEMENT: BEGIN
INSERT INTO user_audit (user_id, action, timestamp)
VALUES (NEW.id, 'INSERT', NOW());
END
ACTION_ORIENTATION: ROW
ACTION_TIMING: AFTER
ACTION_REFERENCE_OLD_TABLE: NULL
ACTION_REFERENCE_NEW_TABLE: NULL
ACTION_REFERENCE_OLD_ROW: OLD
ACTION_REFERENCE_NEW_ROW: NEW
CREATED: 2023-05-25 11:18:37.63
SQL_MODE:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_
BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
DEFINER: root@localhost
CHARACTER_SET_CLIENT: utf8
COLLATION_CONNECTION: utf8_general_ci
DATABASE_COLLATION: latin1_swedish_ci
1 row in set (0.00 sec)
trigger_schema 库
trigger_name 触发器名称
6、删除触发器
#DROP TRIGGER trigger_name;
mysql> DROP TRIGGER insert_user_audit;
Query OK, 0 rows affected (0.00 sec)
#快速的清空表：truncate table table_name;
mysql> truncate table users;
Query OK, 0 rows affected (0.00 sec)
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
mysql> use wing
Database changed
mysql> CREATE TABLE student_info ( stu_id INT ( 11 ) NOT NULL AUTO_INCREMENT, stu_name
VARCHAR ( 255 ) DEFAULT NULL, PRIMARY KEY ( stu_id ) );
Query OK, 0 rows affected (0.01 sec)
mysql> CREATE TABLE student_count (student_count INT(11) DEFAULT 0);
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO student_count VALUES(0);
Query OK, 1 row affected (0.00 sec)
mysql> select * from student_info;
Empty set (0.00 sec)
#创建增加和减少学⽣数量的触发器
mysql> 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 (0.01 sec)
mysql> 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：
mysql> 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: 2023-05-25 11:46:08.77
sql_mode:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_
BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
Definer: root@localhost
character_set_client: utf8
collation_connection: utf8_general_ci
Database Collation: latin1_swedish_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: 2023-05-25 11:46:37.16
sql_mode:
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_
BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
Definer: root@localhost
character_set_client: utf8
collation_connection: utf8_general_ci
Database Collation: latin1_swedish_ci
2 rows in set (0.00 sec)
#查看建好的student_info表，⽬前是空的
mysql> select * from student_info;
Empty set (0.00 sec)
#往⾥⾯插⼊⼀条数据；
mysql> insert student_info values(1,'wing');
Query OK, 1 row affected (0.00 sec)
#查看student_info表⾥⾯已经有了⼀条信息
mysql> select * from student_info;
+--------+----------+
| stu_id | stu_name |
+--------+----------+
| 1 | wing |
+--------+----------+
1 row in set (0.00 sec)
#查看student_count⾥⾯的统计数，
mysql> select * from student_count;
+---------------+
| student_count |
+---------------+
| 1 |
+---------------+
1 row in set (0.00 sec)