MySQL视图介绍

视图定义

视图是一种有结构的虚拟表，本身不存放数据，视图中数据来源于真实的表，真实的表也被称之为基表。

视图作用

1.简化SQL语句: 比如将一条多表联合查询做成视图，后续直接SELECT * 视图进行查询即可。
2.相对安全的权限控制: 针对不同应用只需要建立对应视图表，提供所需要的字段信息，应用无法感知到其他字段信息。
3.保障数据安全: 对视图进行删除操作不会影响基表的数据。

创建视图

基本语法

CREATE
    [OR REPLACE]
    [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}]
    [DEFINER = user]
    [SQL SECURITY { DEFINER | INVOKER }]
    VIEW view_name [(column_list)]
    AS select_statement
    [WITH [CASCADED | LOCAL] CHECK OPTION]

视图算法

ALGORITHM = UNDEFINED | MERGE | TEMPTABLE

UNDEFINED # 未定义(默认的), 由系统自行调整

MERGE # 合并算法: 系统应该先将视图对应的select语句与外部查询视图的select语句进行合并,然后执行(效率高: 常态)

TEMPTABLE # 临时表算法: 系统应该先执行视图的select语句,后执行外部查询语句

create view 视图名字 as select语句

select 语句可以是普通查询、连接查询、联合查询、子查询等各种类型。

创建单表视图: 基表只有一个

[root@GreatSQL][test]>create view v_t_name as select * from t_name;
Query OK, 0 rows affected (0.14 sec)

[root@GreatSQL][test]>show create view v_t_name\G;
*************************** 1. row ***************************
                View: v_t_name
         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v_t_name` AS select `t_name`.`id` AS `id`,`t_name`.`name` AS `name` from `t_name`
character_set_client: utf8
collation_connection: utf8_general_ci
1 row in set (0.01 sec)

ERROR:
No query specified


[root@GreatSQL][test]>desc v_t_name;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   |     | 0       |       |
| name  | varchar(64) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

创建多表视图: 基表来源至少两个

[root@GreatSQL][test]>create view v_t_name_user(id,name,email,age) as select t_name.id,t_name.name,t_user.email,t_user.age from t_name,t_user where t_name.id=t_user.id;
Query OK, 0 rows affected (0.04 sec)

[root@GreatSQL][test]>show create view v_t_name_user\G;
*************************** 1. row ***************************
                View: v_t_name_user
         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v_t_name_user` (`id`,`name`,`email`,`age`) AS select `t_name`.`id` AS `id`,`t_name`.`name` AS `name`,`t_user`.`email` AS `email`,`t_user`.`age` AS `age` from (`t_name` join `t_user`) where (`t_name`.`id` = `t_user`.`id`)
character_set_client: utf8
collation_connection: utf8_general_ci
1 row in set (0.01 sec)

ERROR:
No query specified


[root@GreatSQL][test]>desc v_t_name_user;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int          | NO   |     | 0       |       |
| name  | varchar(64)  | YES  |     | NULL    |       |
| email | varchar(255) | YES  |     | NULL    |       |
| age   | tinyint      | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

使用视图

使用视图主要是为了查询: 将视图当做表一样查询即可

[root@GreatSQL][test]>select * from v_t_name;
+------------+------+
| id         | name |
+------------+------+
|          1 | chen |
| 2147483646 | wu   |
| 2147483647 | ling |
+------------+------+
3 rows in set (0.00 sec)

修改视图

视图本身不可修改, 但是视图的来源是可以修改的.

修改视图: 修改视图本身的来源语句(select语句)

alter view 视图名字 as 新的select语句;

[root@GreatSQL][test]>alter view v_t_name as select t_name.name from t_name;
Query OK, 0 rows affected (0.05 sec)

(Wed Dec 15 01:53:44 2021)[root@GreatSQL][test]>select * from v_t_name;
+------+
| name |
+------+
| chen |
| wu   |
| ling |
+------+
3 rows in set (0.00 sec)

删除视图

[root@GreatSQL][test]>drop view v_t_name;
Query OK, 0 rows affected (0.04 sec)

视图写入数据

单表视图测试写入

[root@GreatSQL][test]>insert into v_t_name values(5,'long');
Query OK, 1 row affected (0.02 sec)

(Wed Dec 15 03:27:17 2021)[root@GreatSQL][test]>select * from t_name;
+------------+------+
| id         | name |
+------------+------+
|          1 | chen |
|          4 | wang |
|          5 | long |
| 2147483646 | wu   |
| 2147483647 | ling |
+------------+------+
5 rows in set (0.00 sec)

# 部分字段的视图也可以写入
[root@GreatSQL][test]>create view v_t_name2 as select t_name.id from t_name;
Query OK, 0 rows affected (0.04 sec)

(Wed Dec 15 03:29:11 2021)[root@GreatSQL][test]>desc v_t_name2;
+-------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------+------+-----+---------+-------+
| id    | int  | NO   |     | 0       |       |
+-------+------+------+-----+---------+-------+
1 row in set (0.00 sec)

(Wed Dec 15 03:29:17 2021)[root@GreatSQL][test]>insert into v_t_name2 values(6);
Query OK, 1 row affected (0.02 sec)

(Wed Dec 15 03:29:29 2021)[root@GreatSQL][test]>select * from t_name;
+------------+------+
| id         | name |
+------------+------+
|          1 | chen |
|          4 | long |
|          5 | long |
|          6 | NULL |
| 2147483646 | wu   |
| 2147483647 | ling |
+------------+------+
6 rows in set (0.00 sec)

多表视图测试写入

# 联合视图不能写入数据
(Wed Dec 15 03:54:59 2021)[root@GreatSQL][test]>insert into v_t_name_user values(100,'chen','123456789@qq.com','16');
ERROR 1394 (HY000): Can not insert into join view 'test.v_t_name_user' without fields list

视图更新数据

单表视图更新数据

[root@GreatSQL][test]>update v_t_name set name='yang' where id='6';
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(Wed Dec 15 03:59:12 2021)[root@GreatSQL][test]>select * from v_t_name;
+------------+------+
| id         | name |
+------------+------+
|          1 | chen |
|          4 | long |
|          5 | long |
|          6 | yang |
| 2147483646 | wu   |
| 2147483647 | ling |
+------------+------+
6 rows in set (0.00 sec)

多表视图更新数据

[root@GreatSQL][test]>select * from v_t_name_user;
+---------+------+------------------+------+
| id      | name | email            | age  |
+---------+------+------------------+------+
| 1000001 | he   | 123456789@qq.com |   12 |
+---------+------+------------------+------+
1 row in set (0.00 sec)

(Wed Dec 15 04:02:22 2021)[root@GreatSQL][test]>update v_t_name_user set name='ma' where id='1000001';
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(Wed Dec 15 04:02:55 2021)[root@GreatSQL][test]>select * from v_t_name_user;
+---------+------+------------------+------+
| id      | name | email            | age  |
+---------+------+------------------+------+
| 1000001 | ma   | 123456789@qq.com |   12 |
+---------+------+------------------+------+
1 row in set (0.00 sec)

视图删除数据

单表视图删除数据

[root@GreatSQL][test]>delete from v_t_name where id=1;
Query OK, 1 row affected (0.02 sec)

(Wed Dec 15 04:07:03 2021)[root@GreatSQL][test]>select * from t_name;
+------------+------+
| id         | name |
+------------+------+
|          4 | long |
|          5 | long |
|          6 | yang |
|     100001 | he   |
|    1000001 | ma   |
| 2147483646 | wu   |
| 2147483647 | ling |
+------------+------+
7 rows in set (0.00 sec)

多表视图删除数据

# 多表联合视图无法执行删除操作
[root@GreatSQL][test]>delete from v_t_name_user where id=1000001;
ERROR 1395 (HY000): Can not delete from join view 'test.v_t_name_user'

Enjoy GreatSQL 😃