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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	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,majornameFROM stuinfo sINNER JOIN major m ON s.`majorid`= m.`id`WHERE s.`stuname` LIKE '张%';

创建视图

CREATE VIEW v1ASSELECT stuname,majornameFROM stuinfo sINNER JOIN major m ON s.`majorid`= m.`id`;

查询姓张的学生名和专业名 (视图方式)

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

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 myv1ASSELECT last_name,department_name,job_titleFROM employees eJOIN departments d ON e.department_id  = d.department_idJOIN 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 myv2ASSELECT AVG(salary) ag,department_idFROM employeesGROUP BY department_id;#②使用SELECT myv2.`ag`,g.grade_levelFROM myv2JOIN job_grades gON 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 myv3ASSELECT * FROM myv2 ORDER BY ag LIMIT 1;SELECT d.*,m.agFROM myv3 mJOIN departments dON 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

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 myv3ASSELECT AVG(salary),job_idFROM employeesGROUP BY job_id;

方式二：

/*语法：alter view 视图名as 查询语句;*/ALTER VIEW myv3ASSELECT * 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 myv1ASSELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"FROM employees;CREATE OR REPLACE VIEW myv1ASSELECT last_name,emailFROM 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';

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 allCREATE OR REPLACE VIEW myv1ASSELECT MAX(salary) m,department_idFROM employeesGROUP 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 myv2ASSELECT 'john' NAME;SELECT * FROM myv2;#更新UPDATE myv2 SET NAME='lucy';#③Select中包含子查询CREATE OR REPLACE VIEW myv3ASSELECT 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;#④joinCREATE OR REPLACE VIEW myv4ASSELECT last_name,department_nameFROM employees eJOIN departments dON 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 myv5ASSELECT * 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 myv6ASSELECT last_name,email,salaryFROM employeesWHERE 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';

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