MySQL 视图、函数、存储过程、触发器、事件（了解即可）

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1.视图：view

视图就是一张虚拟的表。表是真正存数据的，视图只是显示查询结果。
视图的作用：隐藏表的结构、简化sql嵌套查询操作

注意：视图就是你要查询数据的一个中间结果集，我们一般只用来做数据查询的

创建视图：create view view_name as 查询语句

例如：

mysql> create view v_name_course_socre as select  st.name,co.course,sc.score  from students st inner join scores sc on st.StuID=sc.StuID inner join courses co on  sc.courseid=co.courseid;
Query OK, 0 rows affected (1.63 sec)

mysql> show tables;
+---------------------+
| Tables_in_hellodb   |
+---------------------+
| classes             |
| coc                 |
| courses             |
| scores              |
| students            |
| teachers            |
| toc                 |
| v_name_course_socre |
+---------------------+
8 rows in set (0.00 sec)

mysql> select * from v_name_course_socre;
+-------------+----------------+-------+
| name        | course         | score |
+-------------+----------------+-------+
| Shi Zhongyu | Kuihua Baodian |    77 |
| Shi Zhongyu | Weituo Zhang   |    93 |
| Shi Potian  | Kuihua Baodian |    47 |
| Shi Potian  | Daiyu Zanghua  |    97 |
| Xie Yanke   | Kuihua Baodian |    88 |
| Xie Yanke   | Weituo Zhang   |    75 |
| Ding Dian   | Daiyu Zanghua  |    71 |
| Ding Dian   | Kuihua Baodian |    89 |
| Yu Yutong   | Hamo Gong      |    39 |
| Yu Yutong   | Dagou Bangfa   |    63 |
| Shi Qing    | Hamo Gong      |    96 |
| Xi Ren      | Hamo Gong      |    86 |
| Xi Ren      | Dagou Bangfa   |    83 |
| Lin Daiyu   | Taiji Quan     |    57 |
| Lin Daiyu   | Jinshe Jianfa  |    93 |
+-------------+----------------+-------+
15 rows in set (1.69 sec)

删除视图：drop view view_name

mysql> show tables;
+---------------------+
| Tables_in_hellodb   |
+---------------------+
| classes             |
| coc                 |
| courses             |
| scores              |
| students            |
| teachers            |
| toc                 |
| v_name_course_socre |
| v_student           |
+---------------------+
9 rows in set (0.00 sec)

mysql> drop view v_student;
Query OK, 0 rows affected (0.00 sec)

查看创建视图时候用的SQL语句

SHOW CREATE VIEW view_name #只能看视图定义
SHOW CREATE TABLE view_name # 可以查看表和视图

查看视图属性信息：show table status like 'xxx'

mysql> show table status  like 'v_%'\G
*************************** 1. row ***************************
           Name: v_name_course_socre
         Engine: NULL
        Version: NULL
     Row_format: NULL
           Rows: NULL
 Avg_row_length: NULL
    Data_length: NULL
Max_data_length: NULL
   Index_length: NULL
      Data_free: NULL
 Auto_increment: NULL
    Create_time: NULL
    Update_time: NULL
     Check_time: NULL
      Collation: NULL
       Checksum: NULL
 Create_options: NULL
        Comment: VIEW
1 row in set (0.00 sec)

修改视图：

视图只是一个虚拟的表，本身不存放数据，只是某个select语句的执行结果。
修改视图实际上就是修改后台对应表的数据。

mysql> create view v_student as select * from students;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from v_studet;
ERROR 1146 (42S02): Table 'hellodb.v_studet' doesn't exist
mysql> select * from v_student;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  27 | M      |    NULL |      NULL |
+-------+---------------+-----+--------+---------+-----------+
24 rows in set (0.00 sec)

mysql> delete from v_student where StuID=24;
Query OK, 1 row affected (0.11 sec)

mysql> select * from students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL |
+-------+---------------+-----+--------+---------+-----------+
23 rows in set (0.00 sec)

2.MySQL函数：FUNCTION

MySQL的函数分为内置函数和自定义函数。

内置函数查看：

https://dev.mysql.com/doc/refman/8.0/en/sql-function-reference.html
https://dev.mysql.com/doc/refman/5.7/en/sql-function-reference.html

自定义函数：

create function fun_name((parameter_name data_type,...,parameter_name data_type)；

注意：

begin-end用于定义一组语句块
delimiter：mysql的分隔符，mysql客户端中默认是分号（；）。告诉mysql解释器，该段命令是否已经结束了，mysql是否可以执行了

例如：

mysql> DELIMITER //
mysql> CREATE FUNCTION deleteById(id SMALLINT UNSIGNED) RETURNS VARCHAR(20) 
    -> BEGIN
    ->  DELETE FROM students WHERE stuid = id;
    ->  RETURN (SELECT COUNT(*) FROM students);
    -> END//
ERROR 1304 (42000): FUNCTION deleteById already exists
mysql> DELIMITER ;
mysql> select deleteById(23);
+----------------+
| deleteById(23) |
+----------------+
| 22             |
+----------------+
1 row in set (0.01 sec)

RETURNS VARCHAR(20) ：定义返回值的，定义函数的输出数据的类型

3.PROCEDURE 存储过程

作用：和函数的功能差不多，但是函数不能单独作为一个命令来执行。存储过程可以单独作为一个命令来进行执行。

格式：call 需要调用的存储过程

例如：

mysql> delimiter //
mysql> CREATE PROCEDURE selectById(IN id SMALLINT UNSIGNED)
    -> BEGIN
    ->  SELECT * FROM students WHERE stuid = id;
    -> END//
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;
mysql> select * from students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
+-------+---------------+-----+--------+---------+-----------+
22 rows in set (0.00 sec)

mysql> call selectById(2);
+-------+------------+-----+--------+---------+-----------+
| StuID | Name       | Age | Gender | ClassID | TeacherID |
+-------+------------+-----+--------+---------+-----------+
|     2 | Shi Potian |  22 | M      |       1 |         7 |
+-------+------------+-----+--------+---------+-----------+
1 row in set (0.00 sec)

4.TRIGGER 触发器：监控某件事满足条件以后自动执行一些事情

主要是针对数据库据表里的增删改操作，当执行这些操作的时候就触发一个行为。

5.Event 事件

类似于linux里面的计划任务，再某个时间点或者周期执行对应的操作。

注意：事件默认没有开启

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