MySQL--视图 21
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
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
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
#一、创建视图
语法:
create view 视图名 as 查询语句;
#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;
案例:
SELECT * FROM myv3 CREATE OR REPLACE VIEW myv3 AS SELECT AVG(salary),job_id FROM employees GROUP BY job_id;
#方式二:
alter view 视图名 as 查询语句;
案例:
ALTER VIEW myv3 AS SELECT * FROM employees;
#三、删除视图
语法:drop view 视图名,视图名,...;
#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,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('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';