视图

一、MySQL视图

1.1.视图的含义

视图是一个虚拟表，是从数据库中一个或是多个表导出来的表。视图也可以从已经存在的视图基础上定义。
单表视图可以进行增、删、改。对视图的操作语法和操作表一致。
联表视图无法进行DML

1.1.1 视图基本操作:

查看MySQL所有的视图

mysql> show table status where comment='view'G

创建视图

mysql> create ALGORITHM=[MERGE/TEMPTABLE/UNDEFINED] 
view view_name  as  (select * from test);
或是：
mysql> create view view_name(v_id,v_class) as (select s_id,class from stu_info);

补充：

视图的ALGORITHM
ALGORITHM = MERGE/TEMPTABLE/UNDEFINED

MERGE:当使用视图时，会把查询视图的语句和创建视图的语句合并起来，形成一条语句，最后再从基表中查询

TEMPTABLE:当使用视图时，会把创建视图的语句的查询结果当成一张临时表，再从临时表中进行筛选

UNDEFINED:未定义，自动，让系统帮你选

视图的检查:

mysql> show table status like 'view_name'G
或是：
mysql> desc [describe]  view_name;

视图的修改：

mysql> alter view view_name as (select * from test01);
或是：
mysql> create or replace view view_name as (select * from test);

视图的删除：

mysql> drop view view_name;

1.2.视图案例

有两张基本表：student和stu_info，按要求只公布id号、姓名和班级。

表结构如下：

表	字段1	字段2	字段3
student	s_id	name	——
stu_info	s_id	class	addr

1.2.1.创建表和数据

mysql> create table student ( s_id int, name varchar(40) );
mysql> insert into student values(1,'tansk'), (2,'tanmy');
mysql> create table stu_info( s_id int, class  varchar(40), addr varchar(90));
mysql> insert into stu_info values('001','1011','gz'),(002,'1012','nn');

查看表数据：

mysql> select * from student;
+------+-------+
| s_id | name  |
+------+-------+
|    1 | tansk |
|    2 | tanmy |
+------+-------+
2 rows in set (0.00 sec)

mysql> select * from stu_info;
+------+-------+------+
| s_id | class | addr |
+------+-------+------+
|    1 | 1011  | gz   |
|    2 | 1012  | nn   |
+------+-------+------+
2 rows in set (0.00 sec)

mysql>

1.2.2.编写查询SQL

注意：一般的都是先写好查询SQL，在进行视图创建。因为视图中引用的就是这一条SQL

方法一（普通查询）：

mysql> select a.s_id,a.name, b.class from student as a,stu_info as b where a.s_id=b.s_id;
+------+-------+-------+
| s_id | name  | class |
+------+-------+-------+
|    1 | tansk | 1011  |
|    2 | tanmy | 1012  |
+------+-------+-------+
2 rows in set (0.04 sec)

mysql>

方法二（外连接，推荐使用）：

mysql> select a.s_id,a.name, b.class from student as a left join stu_info as b on a.s_id=b.s_id ;
+------+-------+-------+
| s_id | name  | class |
+------+-------+-------+
|    1 | tansk | 1011  |
|    2 | tanmy | 1012  |
+------+-------+-------+
2 rows in set (0.00 sec)

mysql>

1.2.3.创建视图

mysql> create view  view_stuAndinfo_01  as  ( select a.s_id,a.name, b.class from student as a,stu_info as b where a.s_id=b.s_id); 
或是：
mysql> create view  view_stuAndinfo_02  as  (select a.s_id,a.name, b.class from student as a left join stu_info as b on a.s_id=b.s_id);

查询视图：

mysql>  select * from  view_stuAndinfo_01;       
+------+-------+-------+
| s_id | name  | class |
+------+-------+-------+
|    1 | tansk | 1011  |
|    2 | tanmy | 1012  |
+------+-------+-------+
2 rows in set (0.03 sec)

mysql>  select * from  view_stuAndinfo_02;
+------+-------+-------+
| s_id | name  | class |
+------+-------+-------+
|    1 | tansk | 1011  |
|    2 | tanmy | 1012  |
+------+-------+-------+
2 rows in set (0.00 sec)

mysql>

1.2.4.查看视图属性

mysql> show table status like 'view_stuAndinfo_01'G
*************************** 1. row ***************************
           Name: view_stuAndinfo_01
         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)

mysql>

1.3 视图的作用

提高了重用性，就和函数一样 --当频繁获取一些数据的时候，减少了写SQL的过程

提高了安全性。对不同的用户创建不同的视图，每个用户跟进可以看到的信息不同

简化查询，提高了查询性能

提高灵活性，在生产中如果修改表结构导致工作量比较大，可以使用视图（虚拟表）的形式达到修改表结构的效果。

水果大佬