2 案例2：视图的基本使用
2.1 问题
把/etc/passwd文件的内容存储到db9库下的user表里
添加新字段id 存储记录的行号(在所有字段的前边)
创建视图v1 结构及数据user表的字段、记录一样。
创建视图v2 只有user表shell是/bin/bash用户信息 。
分别对视图表和基表执行insert update delete 操作。
删除视图v1 和 v2
2.2 步骤
实现此案例需要按照如下步骤进行。
步骤一：视图的基本使用
什么是视图：是一种虚拟存在的表
内容与真实的表相似，包含一系列带有名称的列和行数据。
视图并不在数据库中以存储的数据的形式存在。
行和列的数据来自定义视图时查询所引用的基本表，并且在具体引用视图时动态生成。
更新视图的数据，就是更新基表的数据
更新基表数据，视图的数据也会跟着改变
1）把/etc/passwd文件的内容存储到db9库下的user表里
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[root@mysql51 ~]# mysql -u root -p123456
mysql> create database db9;
Query OK, 1 row affected (10.00 sec)
mysql> create table db9.user(username char(20),password char(1),uid \
int(2),gid int(2),comment char(100),homedir char(100),shell char(50));
//创建存储数据的表结构
Query OK, 0 rows affected (0.02 sec)
[root@mysql51 ~]# cp /etc/passwd /var/lib/mysql-files/
[root@mysql51 ~]# ls /var/lib/mysql-files/
passwd
mysql> load data infile "/var/lib/mysql-files/passwd" into table db9.user fields terminated by ":" lines terminated by "\n"; //导入文件内容到db9.user
Query OK, 41 rows affected (0.02 sec)
Records: 41 Deleted: 0 Skipped: 0 Warnings: 0
2）添加新字段id 存储记录的行号(在所有字段的前边)
mysql> alter table db9.user add id int(2) primary key auto_increment first;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> use db9;
mysql> desc user;
+----------+-----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-----------+------+-----+---------+----------------+
| id | int(2) | NO | PRI | NULL | auto_increment |
| username | char(20) | YES | | NULL | |
| password | char(1) | YES | | NULL | |
| uid | int(2) | YES | | NULL | |
| gid | int(2) | YES | | NULL | |
| comment | char(100) | YES | | NULL | |
| homedir | char(100) | YES | | NULL | |
| shell | char(50) | YES | | NULL | |
+----------+-----------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
3）创建视图v1 结构及数据user表的字段、记录一样
mysql> create view v1 as select * from user;
Query OK, 0 rows affected (0.00 sec)
4）创建视图v2 只有user表shell是/bin/bash用户信息
mysql> create view v2 as select shell from user;
Query OK, 0 rows affected (0.01 sec)
5）分别对视图表和基表执行insert update delete 操作
mysql> insert into v1(username,uid) values("jarry",9); //插入记录
Query OK, 1 row affected (0.00 sec)
mysql> update v1 set uid=9 where username="adm"; //更新记录
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> delete from v1 where uid=9; //删除记录
Query OK, 2 rows affected (0.01 sec)
6）删除视图v1 和 v2
mysql> drop view v1;
Query OK, 0 rows affected (0.00 sec)
mysql> drop view v2;
Query OK, 0 rows affected (0.00 sec)
注意：对视图操作即是对基本操作，反之亦然！！！
3 案例3：视图进阶操作
3.1 问题
练习OR REPLACE的选项使用
练习WITH LOCAL CHECK OPTION 选项的使用
练习WITH CASCADED CHECK OPTION 选项的使用
3.2 步骤
实现此案例需要按照如下步骤进行。
步骤一：视图进阶操作
1）创建视图完全格式
mysql> create table user2 select username,uid,gid from user limit 3;
//快速建表（user2表）
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> create table info select username,uid,homedir,shell from user limit 5;
//快速建表（info表）
Query OK, 5 rows affected (0.02 sec)
Records: 5 Duplicates: 0 Warnings: 0
查询user2.username=info.username的字段
mysql> select * from user2 left join info on user2.username=info.username;
+----------+------+------+----------+------+---------+---------------+
| username | uid | gid | username | uid | homedir | shell |
+----------+------+------+----------+------+---------+---------------+
| root | 0 | 0 | root | 0 | /root | /bin/bash |
| bin | 1 | 1 | bin | 1 | /bin | /sbin/nologin |
| daemon | 2 | 2 | daemon | 2 | /sbin | /sbin/nologin |
+----------+------+------+----------+------+---------+---------------+
3 rows in set (0.00 sec)
2）关联查询建的视图 默认不允许修改视图字段的值
mysql> create view v4 as select * from user2 left join info on user2.username=info.username; //创建失败
ERROR 1060 (42S21): Duplicate column name 'username'
mysql> create view v4 as select a.username as ausername,b.username as busername, a.uid as auid,b.uid as buid from user2 a left join info b on a.username=b.username;
//创建成功
Query OK, 0 rows affected (0.00 sec)
mysql> select * from v4;
+-----------+-----------+------+------+
| ausername | busername | auid | buid |
+-----------+-----------+------+------+
| root | root | 0 | 0 |
| bin | bin | 1 | 1 |
| daemon | daemon | 2 | 2 |
+-----------+-----------+------+------+
3 rows in set (0.00 sec)
mysql> desc v4;
+-----------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+----------+------+-----+---------+-------+
| ausername | char(20) | YES | | NULL | |
| busername | char(20) | YES | | NULL | |
| auid | int(2) | YES | | NULL | |
| buid | int(2) | YES | | NULL | |
+-----------+----------+------+-----+---------+-------+
4 rows in set (0.00 sec)
3）OR REPLACE的选项使用
创建时，若视图已存在，会替换已有的视图
语法格式：create or replace view视图名as select 查询； //达到修改已有视图的目的
mysql> create or replace view v4 as select a.username as ausername,b.username as busername, a.uid as auid,b.uid as buid from user2 a left join info b on a.username=b.username;
Query OK, 0 rows affected (0.00 sec)
4）WITH LOCAL CHECK OPTION
LOCAL和CASCADED关键字决定检查的范围
LOCAL 仅检查当前视图的限制
CASCADED 同时要满足基表的限制（默认值）
mysql> create table user1 select username,uid,shell from user where uid>=5 and uid <=40;
Query OK, 11 rows affected (0.01 sec)
Records: 11 Duplicates: 0 Warnings: 0
mysql> create view v1 as select username,uid from user1 where uid<=20;
Query OK, 0 rows affected (0.01 sec)
mysql> update v1 set uid=21 where username="sync";
//操作超过视图表的条件限制（uid<=20）之后，在视图表里面查看不到，在基表里可以查看到
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update user1 set uid=41 where username="ftp";
//基表在超过条件限制（uid>=5 and uid <=40),在基表里依然可以查看到
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> create table a select * from user where uid < 10;
//快速创建一个新表a
Query OK, 7 rows affected (0.01 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> create view v3 as select * from a where uid < 10 with check option;
//不写默认为CASCADED检查自己和a要满足的要求即可
Query OK, 0 rows affected (0.00 sec)
mysql> update v3 set uid=9 where username="adm"; //更改成功
Query OK, 0 rows affected (0.01 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> create view v2 as select * from v1 where uid >= 5 with local check option;
//满足自身v2的要求
Query OK, 0 rows affected (0.00 sec)
mysql> update v2 set uid=9 where username="sync";
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
5）WITH CASCADED CHECK OPTION
mysql> create view v5 as select * from v1 where uid >= 5 with cascaded check option;
Query OK, 0 rows affected (0.00 sec)
