视图

什么是视图?

  • 视图: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。
  • 应用场景
    • 多个地方用到同样的查询结果
    • 该查询结果使用的sql语句较复杂
  • 案例
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`;

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

视图的创建

  • 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`;

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

视图的修改

  • 语法
  方式一:
create or replace view 视图名 as 查询语句;
方式二:
alter view 视图名 as 查询语句;
  • 案例
#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;

视图的删除

  • 用户可以一次删除一个或者多个视图,前提是必须有该视图的drop权限。
  • 语法:drop view 视图1,视图2,…;
  • 案例
/*
语法:drop view 视图名,视图名,...;
*/
DROP VIEW emp_v1,emp_v2,myv3;

视图的查看

  • 语法
desc 视图名;
show create view 视图名;
  • 案例
DESC myv3;
SHOW CREATE VIEW myv3;

视图的更新

  • 插入:insert
  • 修改:update
  • 删除:delete
  • 查看:select
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';

注意:视图一般用于查询的,而不是更新的,所以具备以下特点的视图都不允许

  • 包含以下关键字的sql语句:分组函数、distinctgroup byhavingunion或者union all
  • 常量视图
  • Select中包含子查询
  • join
  • from一个不能更新的视图
  • where子句的子查询引用了from子句中的表
#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、distinctgroup  byhaving、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';

视图和表的对比 关键字 是否占用物理空间 使用 视图
view 占用较小,只保存sql逻辑 一般用于查询 表 table 保存实际的数据 增删改查

 

posted @ 2020-12-28 16:36  上天安排的最大嘛!  阅读(177)  评论(0编辑  收藏  举报