#视图
/*
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
比如：舞蹈班和普通班级的对比
创建语法的关键字 是否实际占用物理空间 使用
视图 create view 只是保存了sql逻辑，没有结果 增删改查，只是一般不能增删改
表 create table 保存了数据 增删改查
*/
#案例：查询姓张的学生名和专业名
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`; #封装了
SELECT * FROM v1 WHERE stuname LIKE ‘张%‘; # 以后只执行这句ok
#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%‘;
#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`;
#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; #1
SHOW CREATE VIEW myv3; # 2
#五、视图的更新
CREATE OR REPLACE VIEW myv1 AS SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary" FROM employees; #不能更新
CREATE OR REPLACE VIEW myv1 AS SELECT last_name,email FROM employees; #可以更新
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‘;
#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; #更新 SELECT * FROM myv1; UPDATE myv1 SET m=9000 WHERE department_id=10;
#②常量视图 不能更新
CREATE OR REPLACE VIEW myv2 AS SELECT ‘john‘ NAME; #更新 SELECT * FROM myv2; UPDATE myv2 SET NAME=‘lucy‘;
#③Select中包含子查询 不能更新
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 不能更新
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‘; #没有报错 INSERT INTO myv4 VALUES(‘陈真‘,‘xxxx‘);
#⑤from一个不能更新的视图 不能更新
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 不能更新
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‘;
#一、创建视图emp_v1,要求查询电话号码以‘011’开头的员工姓名和工资、邮箱
CREATE OR REPLACE VIEW emp_v1 AS SELECT last_name,salary,email FROM employees WHERE phone_number LIKE ‘011%‘;
#二、创建视图emp_v2，要求查询部门的最高工资高于12000的部门信息
CREATE OR REPLACE VIEW emp_v2 AS SELECT MAX(salary) mx_dep,department_id FROM employees GROUP BY department_id HAVING MAX(salary)>12000; SELECT d.*,m.mx_dep FROM departments d JOIN emp_v2 m ON m.department_id = d.`department_id`;