视图

什么是视图？

• 视图：虚拟表,和普通表一样使用。mysql5.1版本出现的新特性，是通过表动态生成的数据。
• 应用场景
  • 多个地方用到同样的查询结果
  • 该查询结果使用的sql语句较复杂

• 案例

SELECT stuname,majorname FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '张%';

CREATE VIEW v1 AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;

SELECT * FROM v1 WHERE stuname LIKE '张%';

视图的创建

• 语法：create view 视图名 as 查询语句;

USE myemployees;

#1.查询姓名中包含a字符的员工名、部门名和工种信息
    #①创建
    CREATE VIEW myv1 AS
    SELECT last_name,department_name,job_title
    FROM employees e
    JOIN departments d ON e.department_id  = d.department_id
    JOIN jobs j ON j.job_id  = e.job_id;

    #②使用
    SELECT * FROM myv1 WHERE last_name LIKE '%a%';

#2.查询各部门的平均工资级别
    #①创建视图查看每个部门的平均工资
    CREATE VIEW myv2 AS
    SELECT AVG(salary) ag,department_id
    FROM employees GROUP BY department_id;

    #②使用
    SELECT myv2.`ag`,g.grade_level FROM myv2
    JOIN job_grades g
    ON myv2.`ag` BETWEEN g.`lowest_sal` AND g.`highest_sal`;

#3.查询平均工资最低的部门信息
SELECT * FROM myv2 ORDER BY ag LIMIT 1;

#4.查询平均工资最低的部门名和工资
    CREATE VIEW myv3 AS
    SELECT * FROM myv2 ORDER BY ag LIMIT 1;

    SELECT d.*,m.ag FROM myv3 m
    JOIN departments d
    ON m.`department_id`=d.`department_id`;

视图的好处

• 重用sql语句
• 简化复杂的sql操作，不必知道它的查询细节
• 保护数据，提高安全性

视图的修改

• 语法

  方式一：
create or replace view 视图名 as 查询语句;
方式二：
alter view 视图名 as 查询语句;

• 案例

#方式一：
/*
create or replace view  视图名 as 查询语句;
*/
SELECT * FROM myv3 

CREATE OR REPLACE VIEW myv3 AS
SELECT AVG(salary),job_id
FROM employees GROUP BY job_id;

#方式二：
/*
语法：
alter view 视图名 as 查询语句;
*/
ALTER VIEW myv3 AS SELECT * FROM employees;

视图的删除

• 用户可以一次删除一个或者多个视图，前提是必须有该视图的drop权限。
• 语法：drop view 视图1，视图2,…;
• 案例

/*
语法：drop view 视图名,视图名,...;
*/
DROP VIEW emp_v1,emp_v2,myv3;

视图的查看

• 语法

desc 视图名;
show create view 视图名;

• 案例

DESC myv3;
SHOW CREATE VIEW myv3;

视图的更新

• 插入：insert
• 修改：update
• 删除：delete
• 查看：select

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"
FROM employees;

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;

SELECT * FROM myv1;
SELECT * FROM employees;
#1.插入

INSERT INTO myv1 VALUES('张飞','zf@qq.com');

#2.修改
UPDATE myv1 SET last_name = '张无忌' WHERE last_name='张飞';

#3.删除
DELETE FROM myv1 WHERE last_name = '张无忌';

注意：视图一般用于查询的，而不是更新的，所以具备以下特点的视图都不允许

• 包含以下关键字的sql语句：分组函数、distinct、group by、having、union或者union all
• 常量视图
• Select中包含子查询
• join
• from一个不能更新的视图
• where子句的子查询引用了from子句中的表

#具备以下特点的视图不允许更新
#①包含以下关键字的sql语句：分组函数、distinct、group  by、having、union或者union all

CREATE OR REPLACE VIEW myv1
AS
SELECT MAX(salary) m,department_id
FROM employees
GROUP BY department_id;

SELECT * FROM myv1;

#更新
UPDATE myv1 SET m=9000 WHERE department_id=10;

#②常量视图
CREATE OR REPLACE VIEW myv2
AS
SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATE myv2 SET NAME='lucy';

#③Select中包含子查询

CREATE OR REPLACE VIEW myv3
AS
SELECT department_id,(SELECT MAX(salary) FROM employees) 最高工资
FROM departments;

#更新
SELECT * FROM myv3;
UPDATE myv3 SET 最高工资=100000;

#④join
CREATE OR REPLACE VIEW myv4
AS
SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id  = d.department_id;

#更新
SELECT * FROM myv4;
UPDATE myv4 SET last_name  = '张飞' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('陈真','xxxx');

#⑤from一个不能更新的视图
CREATE OR REPLACE VIEW myv5
AS SELECT * FROM myv3;

#更新
SELECT * FROM myv5;
UPDATE myv5 SET 最高工资=10000 WHERE department_id=60;

#⑥where子句的子查询引用了from子句中的表

CREATE OR REPLACE VIEW myv6
AS
SELECT last_name,email,salary
FROM employees
WHERE employee_id IN(
    SELECT  manager_id
    FROM employees
    WHERE manager_id IS NOT NULL
);

#更新
SELECT * FROM myv6;
UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';


视图和表的对比
         关键字          是否占用物理空间             使用
视图      view          占用较小，只保存sql逻辑    一般用于查询
 表      table          保存实际的数据              增删改查