MySQL基础-20视图
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声明:此mysql基础学习源自尚硅谷。(推荐)b站官方链接:https://www.bilibili.com/video/BV1xW411u7ax?p=1
视图
含义:虚拟表,和普通表一样使用
MySQL从5.0.1版本出现的新特性。一种虚拟存在的表,行和列的数据来自定义视图的查询中使用的表,并且是在使用视图时动态生成的,只保存了sql逻辑,不保存查询结果。
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
好处: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
案例:查询姓张的学生名和专业名
-- 原始操作
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE ‘张%‘;
-- 使用视图操作
CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;
-- 查询视图
SELECT * FROM v1 WHERE stuname LIKE ‘张%‘;
1.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`;
2.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;
3.删除视图
语法:
drop view 视图名1,视图名2,...;
DROP VIEW emp_v1,emp_v2,myv3;
4.查看视图
# 方式1
DESC myv3;
# 方式2
SHOW CREATE VIEW myv3;
5.视图的更新
更新了视图中数据,原始表中也会更新视图中的数据
视图一般用于查询的,而不是更新的,一般视图也不会更新数据,由于视图更新限制条件,一般也不能更新
CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;
SELECT * FROM myv1;
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; # 存在group by,不允许更新
SELECT * FROM myv1;
#更新失败
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;
#更新失败
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;
SELECT * FROM myv3;
#更新失败
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;
SELECT * FROM myv4;
#更新
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;
SELECT * FROM myv5;
#更新失败
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
);
SELECT * FROM myv6;
#更新失败
UPDATE myv6 SET salary=10000 WHERE last_name = ‘k_ing‘;
视图与实际表对比
创建语法关键字 | 是否实际占用物理空间 | 使用 | |
---|---|---|---|
视图 | create view | 只保存SQL逻辑 | 增删改查,一般不能增删改 |
表 | create table | 保存数据 | 增上改查 |
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