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
视图（View）是一种虚表，允许用户实现以下几点：
1.用户或用户组查找结构数据的方式更自然或直观。
2.限制数据访问，用户只能看到有限的数据，而不是完整的表。
3.汇总各种表中的数据，用于生成报告。
SQLite 视图是只读的，因此可能无法在视图上执行 DELETE、INSERT 或 UPDATE 语句。但是可以在视图上创建一个触发器，当尝试 DELETE、INSERT 或 UPDATE 视图时触发，需要做的动作在触发器内容中定义。
| 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 |
CREATE VIEW 的基本语法如下：
CREATE [TEMP | TEMPORARY] VIEW view_name AS SELECT column1, column2..... FROM table_name WHERE [condition];
案例：查询姓本的学生名和专业名
SELECT stuname,majorname FROM stuinfo s INNER JOIN major m ON s.`majorid`= m.`id` WHERE s.`stuname` LIKE '%本%'; 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 '%本%';
创建视图
语法：
create view 视图名
as
查询语句;
以下测试请结合数据库 myemployees;
链接：点击
提取码：rhjw
初学者怎么用这个数据库，最简单的就是在图形化工具比如Navicat里执行这个文件
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; #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
USE myemployees; #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;
删除视图
语法：drop view 视图名,视图名,…;
DROP VIEW emp_v1,emp_v2,myv3;
查看视图
DESC myv3; SHOW CREATE VIEW myv3;
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; 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('本','12345@qq.com');
2.修改
UPDATE myv1 SET last_name = '本' WHERE last_name='特';
3.删除
DELETE FROM myv1 WHERE last_name = '本';
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 = '本' 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';
练习
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%';
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`;
