MySQL-トリガ

1、      ：
              ，           ，trigger
2、             ，        ，        、        
3、              ，        、     ，         
4、        MySQL          ，   Java  、Python          
5、     ：
        CREATE：
            [DEFINER = { user | CURRENT_USER }]
            TRIGGER trigger_name
            trigger_time trigger_event
            ON tbl_name FOR EACH ROW
            trigger_body
          ：
            trigger_name：      
            trigger_time：{ BEFORE | AFTER }，            
            trigger_event:：{ INSERT |UPDATE | DELETE }，       
            tbl_name：         
6、     
        SHOW TRIGGERS
             information_schema.triggers         ，          。
        mysql> USE information_schema;
        Database changed
        mysql> SELECT * FROM triggers WHERE
7、     
        DROP TRIGGER trigger_name;
              ：truncate table student_info

         ，     INSERT   ，     ，DELETE   ，     
    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)

                   
    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)

    #          ：
    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: root@localhost
    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: root@localhost
    character_set_client: utf8
    collation_connection: utf8_general_ci
      Database Collation: utf8_general_ci
    2 rows in set (0.00 sec)

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

             ；
    MariaDB [db1]> insert student_info values(1,'cobbler');
    Query OK, 1 row affected (0.01 sec)

      student_info           
    MariaDB [db1]> select * from student_info;
    +--------+----------+
    | stu_id | stu_name |
    +--------+----------+
    |      1 | cobbler  |
    +--------+----------+
    1 row in set (0.00 sec)
      student_count      ，
    MariaDB [db1]> select * from student_count;
    +---------------+
    | student_count |
    +---------------+
    |             1 |
    +---------------+
    1 row in set (0.00 sec)