含义：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
好处：
1、简化sql语句
2、提高了sql的重用性
3、保护基表的数据，提高了安全性
#一、创建视图
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%';
#二、视图的修改
#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;
#三、删除视图
语法：
drop view 视图名,视图名,...;
DROP VIEW emp_v1,emp_v2,myv3;
#四、查看视图
DESC myv3;
SHOW CREATE VIEW myv3;
#五、视图的更新
CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
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';
#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';
七、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
