MySQL之视图
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
比如:
舞蹈班和普通班级的对比
优点:
- 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
视图和表的对比:
| 创建语法 | 是否实际占用物理空间 | 使用 | |
|---|---|---|---|
| 视图 | create view | 没有,只是保存了sql逻辑 | 增删改查,一般不能增删改 |
| 表 | create table | 占用,保存了数据 | 增删改查 |
案例:查询姓张的学生名和专业名
use students;
drop table if exists stuinfo;
create table if not exists stuinfo(
id int,
stuname varchar(20),
gender char(1),
age int default 18,
seat int,
majorid int
) default charset=utf8;
drop table if exists major;
create table major(
id int,
majorName varchar(20)
);
select stuname,majorname
from stuinfo s
inner join major m on s.majorid=m.id
where s.stuname like '张%';
create view myv1
as
select stuname,majorname
from stuinfo s
inner join major m on s.majorid=m.id;
select * from myv1 where stuname like '张%';
一.创建视图
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;
二、视图的修改
方式一：
create or replace view 视图名
as
查询语句;
use myemployees;
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;
alter view myv3
as
select * from employees;
select * from myv3;
三.删除视图
drop view 视图名,视图名,...;
drop view myv1,myv2,myv3;
四.查看视图
desc myv3;
show create view myv3;
五.视图的更新(可以更新原始表)
create or replace view myv1
as
select last_name,email
from employees;
select * from myv1;
1.插入
insert into myv1 values('张飞','[email protected]');
2.修改
set SQL_SAFE_UPDATES = 0;
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
- 包含以下关键字的sql语句：
分组函数、distinct、group by、having、union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
drop view myv1;
①group by
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; # 失败error1288
②常量视图
create or replace view myv2
as
select 'john' name;
select * from myv2;
更新
update myv2 set name='lucy'; # 失败error1288
③Select中包含子查询
create or replace view myv3
as
select (select max(salary) from employees) 最高工资;
```s
更新
```sql
select * from myv3;
update myv3 set 最高工资=10000; # 失败error1288
练习题
一、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%';
select * from emp_v1;
二、要求将视图emp_v1修改为查询电话号码以‘011’开头的并且邮箱中包含e字符的员工姓名和邮箱、电话号码
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.*,emp_v2.mx_dep
from departments d
join emp_v2
on emp_v2.department_id=d.department_id;