mysql——视图

视图

含义：虚拟表，和普通表一样使用，
mysql5.1开始出现的新版本，通过普通表动态生成的数据,
行和列的数据来自定义视图的查询中使用的表,
只保存了SQL逻辑，不保存查询结果

特点：①临时性②重复使用性

优点：
1、重用SQL语句
2、简化复杂的SQL操作，不必知道它的查询细节
3、保护数据，提高安全性

视图和表的对比

	创建的关键字	是否占用实际的物理空间	使用
视图	create view	只保存sql逻辑	增删改查，一般不进行这样操作
表	create table	保存数据	增删改查

案例：查询姓张的学生名和专业名
USE student;
SELECT stuname,majorname
FROM stuinfo s
JOIN major m ON s.majorid=m.id
WHERE stuname='张%';

CREATE VIEW tab_s_m AS
SELECT studentname,majorname
FROM student s
JOIN major m ON s.majorid=m.majorid;

SELECT * FROM tab_s_m
WHERE studentname LIKE '张%';

一、创建视图

语法：
CREATE  VIEW 视图名
AS 
查询语句;案例1.查询姓名中包含a字符的员工名、部门名和工种信息

USE myemployees;
DROP VIEW ainfo;
CREATE VIEW ainfo
AS
SELECT last_name,department_name,j.*
FROM employees e
JOIN departments d ON e.department_id=d.department_id
JOIN jobs j ON e.job_id=j.job_id;

SELECT * FROM ainfo
WHERE last_name LIKE '%a%';

案例2.查询各部门的平均工资级别

CREATE VIEW avgs_table
AS 
SELECT AVG(Salary) avgs,department_id
FROM employees
GROUP BY department_id;

SELECT department_id,grade_level
FROM avgs_table 
JOIN job_grades ON avgs_table.avgs BETWEEN lowest_sal AND highest_sal

案例3.查询平均工资最低的部门信息

SELECT *
FROM avgs_table
ORDER BY avgs ASC
LIMIT 1;

案例4.查询平均工资最低的部门名和工资

SELECT d.*,ag.avgs
FROM departments d
JOIN avgs_table ag ON ag.department_id=d.department_id
WHERE d.department_id=(
    SELECT department_id
    FROM avgs_table
    ORDER BY avgs ASC
    LIMIT 1
);

二、视图的修改

方式一：
    语法：
        CREATE OR REPLACE VIEW 视图名
        AS
        查询语句;

方式二：
    语法：
        ALTER VIEW 视图名
        AS
        查询语句;

三、删除视图

DROP VIEW 视图名,视图名......

DROP VIEW emp_v1,emp_v2,test_v1;

四、查看视图

CREATE VIEW test_v1
AS
SELECT * FROM employees;

DESC test_v1;
SHOW CREATE VIEW test_v1;

1、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 VIEW emp_v1
AS
SELECT phone_number,last_name,Salary,email 
FROM employees;

SELECT *
FROM emp_v1
WHERE phone_number LIKE '011%';

2、要求将视图 emp_v1 修改为查询电话号码以‘011’开头的并且邮箱中包含 e 字符的员工
姓名和邮箱、电话号码

ALTER VIEW emp_v1
AS
SELECT last_name,email,phone_number
FROM employees
WHERE phone_number LIKE '011%'
AND email LIKE '%e%';

SELECT *
FROM emp_v1;

CREATE OR REPLACE VIEW emp_v1
AS
SELECT last_name,email,phone_number
FROM employees
WHERE phone_number LIKE '011%'
AND email LIKE '%e%';

SELECT *
FROM emp_v1;

6、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 d.*
FROM employees e
JOIN departments d ON e.department_id=d.department_id
GROUP BY e.department_id
HAVING MAX(e.salary)>12000;

SELECT * FROM emp_v2;

五、视图的更新

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

1、插入数据

SELECT * FROM myv1;
SELECT * FROM employees;
INSERT INTO myv1 VALUES('张飞','[email protected]');#对视图插入数据同时将数据插入到原始表中

2、修改数据

UPDATE myv1 SET last_name='张无忌' WHERE last_name='张飞';

3、删除数据

DELETE FROM myv1 WHERE last_name='张无忌';

视图的可更新性和视图中查询的定义有关系，以下类型的视图是不能更新的。
• 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 test_v1
AS
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id;

SELECT * FROM test_v1;

INSERT INTO test_v1 VALUES(10000,120);

•常量视图

CREATE OR REPLACE VIEW test_v2
AS
SELECT 'john' NAME;
SELECT * FROM test_v2;

UPDATE test_v2 SET NAME='little baby' WHERE NAME='john';

• SELECT中包含子查询

CREATE OR REPLACE VIEW test_v3
AS
SELECT (SELECT MAX(Salary) FROM employees) 最高工资;
SELECT * FROM test_v3;

UPDATE test_v3 SET 最高工资=12000;

• JOIN

CREATE OR REPLACE VIEW test_v4
AS
SELECT last_name,department_name,j.*
FROM employees e
JOIN departments d ON e.department_id=d.department_id
JOIN jobs j ON e.job_id=j.job_id;

SELECT * FROM test_v4 WHERE last_name='little baby';
UPDATE test_v4 SET department_name='little baby' WHERE job_id='AC_MGR';#join可以update
INSERT INTO test_v4(last_name) VALUES('little baby');    #执行正常，但结果未插入

• FROM一个不能更新的视图

CREATE OR REPLACE VIEW test_v5
AS
SELECT * FROM test_v3

SELECT * FROM test_v5;
UPDATE test_v5 SET 最高工资=12000;

• WHERE子句的子查询引用了FROM子句中的表

CREATE OR REPLACE VIEW test_v6
AS
SELECT last_name,UPDATE test_v3 SET 最高工资=12000;
FROM employees e1
WHERE e1.employee_id IN (
    SELECT e2.manager_id
    FROM employees e2
);

SELECT * FROM test_v6;

INSERT INTO test_v6 VALUES('ll',2000);#可以
DELETE FROM test_v6 WHERE Last_name='ll';#不可以
UPDATE test_v6 SET last_name='xwg' WHERE last_name='ll';#不可以