MySq视图

含义：虚拟表，和普通表一样使用，类似于函数的封装
好处：简化了sql语句，提高了sql的重用性，保护了基表的数据，提高了安全性

一、视图的创建

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

//创建视图
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;
//使用视图
select * from myv1 where last_name like "%a%";

2. 查询各部门的平均工资级别

//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;
//
select myv2."ag",g.grade_level from myv2 join job_grades g on 
myv2."ag" between g."lowest_sal" and g."hihest_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

1. 方式一：

//如果没有就创建，有就替换
create or replace view 视图名
as
查询语句;

create or replace view myv3 as
select avg(salary) ,job_id from employees group by job_id;

2. 方式二：

alter view 视图名 
as 
查询语句;

alter view myv3 as select * from employees;

三、删除视图

语法：
drop view 视图名,视图名,.....;

drop view myv1,myv2,myv3;

四、查看视图

desc 视图名;
show create view 视图名;

1. 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%";

2. 创建视图emp_v2,要求查询部门最高的工资高于12000的部门信息

create or replace view emp_v2 as
select max(salary),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";

五、视图的更新

create or replace view myv1 as 
select last_name,email from employees;
//插入
insert into myv1 values("张飞","[email protected]");		//原始表中也有数据
//修改
update myv1 set last_name= "张无忌" where last_name = "张飞";
//删除
delete from myv1 where last_name = "张无忌";

具备以下特点的视图是不允许更新的：

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;	会报错误

2. 常量视图

create or replace view myv1 as 
select "name";
//更新
update myv1 set name = "Lucy";	会报错误

3. select 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
update myv3 set 最高工资 = "100000";	会报错误

4. join

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
update myv4 set last_name = "张飞" where last_name = "Whalen";	能正常修改
insert into myv4 values("陈真","xxx");	会报错误

5. from 一个不能更新的视图

create or replace view myv5 as select * from myv3;
//更新
update myv5 set 最高工资 = 10000 where department_id = 60;	会报错误

6. where 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
update myv6 set salary = 10000 where last_name = "k-ing";	//会报错误

六、视图和表的对比

		创建语法的关键字	是否占用物理空间		使用
视图：	create view 	只是保存了sql逻辑		增删改查,一般不能增删改,只能用于查询
表：		create table	报错了数据			增删改查

七、delete和truncate在事务使用时的区别

1. 演示delete 能正常回滚

set autocommit = 0;
start transaction;
delete from account;
rollback;

2. 演示truncate 不能正常回滚

set autocommit = 0;
start transaction;
truncate from account;
rollback;