视图 视图 是通过定制的方式显示 一个或者多个表 的数据。
视图 可以视为
“虚拟表”
或
“存储的查询”
。
视图 的优点： 提供了另外一种级别的表安全性 隐藏了数据的复杂性 简化了用户的
SQL
命令 隔离基表结构的改变 通过重命名列，从另一个角度提供数据。
视图里面不存放数据，是在基表上建立的查询，查询的话会从基表把数据取出返回。
创建视图
sqlplus t1/t1@orcl
#查看当前用户所拥有视图
select * from user_views;
SQL> select * from tab;
TNAME TABTYPE CLUSTERID
------------------------------ ------- ----------
STUDENT TABLE
SQL> desc student;
Name Type Nullable Default Comments
----- ----------- -------- ------- --------
SNO NUMBER(4) Y
SNAME VARCHAR2(2) Y
insert into student values(1,'wa');
#创建视图
create view view1 as
select * from student where sno = 1;
select * from user_views;
select * from view1;
更新视图
#更新视图
update view1 set sno = 5 where sno = 1;
commit;
#注意注意！
！
！
！
更新了视图表对应被更新了！
！
！
！
SQL> select * from student;
SNO SNAME
----- -----
5 wa
#视图查不到东西了！
SQL> select * from view1;
SNO SNAME
----- -----
with check option
#创建视图
create view view2 as
select * from student where sno = 5 with check option;
#查询视图
SQL> select * from view2;
SNO SNAME
----- -----
5 wa
#不让修改！
with check optin为了不让减少视图结果集的操作
SQL> update view2 set sno = 10 where sno = 5;
update view2 set sno = 10 where sno = 5
ORA-01402: view WITH CHECK OPTION where-clause violation
order by
SQL> create view view3 as
select * from student order by sno desc;
SQL> select * from view3;
SNO SNAME
----- -----
7 C
6 B
5 A
5 wa
链接视图
SQL> select * from student;
SNO SNAME
----- -----
1 B
2 C
SQL> select * from address;
ID NAME
--------------------------------------- --------------------
1 贵阳
2 大连
create view view_student_address
as
select s.sname,a.name
from student s,address a
where s.sno = a.id;
SQL> select * from view_student_address;
SNAME NAME
----- --------------------
B 贵阳
C 大连
#无法修改与非建值保存表对应的列
#无法修改任何一列，因为数据来自多张表。
#没有键保留表无法更改！
SQL> update view_student_address set name = '河南' where sname = 'B';
update view_student_address set name = '河南' where sname = 'B'
ORA-01779: cannot modify a column which maps to a non key-preserved table
键保留表
drop table student;
drop table department;
create table student(id int,name varchar2(20),deptno varchar2(20));
create table department(deptno varchar2(20),name varchar2(20));
insert into student values(1,'wang','001');
insert into student values(2,'li','001');
insert into student values(3,'zhang','002');
insert into department values('001','人工智能系');
insert into department values('002','软件工程系');
insert into department values('003','ai系');
commit;
drop view view_stu_dept;
create view view_stu_dept
as
select s.id,s.name sname,s.deptno deptno1,d.deptno deptno2,d.name deptname
from student s,department d
where s.deptno = d.deptno;
select * from view_stu_dept;
# 因为student表的ID为主键 可以修改student而不能修改department
# student键保留表 department非键保留表
SQL> update view_stu_dept set sname = 'xiaoyi' where id = 1;
update view_stu_dept set sname = 'xiaoyi' where id = 1
ORA-01779: cannot modify a column which maps to a non key-preserved table
咳咳：翻车了！
问题不大遇到问题需要解决！
ALTER TABLE student ADD PRIMARY KEY(id);
ALTER TABLE department ADD PRIMARY KEY(deptno);
drop view view_stu_dept;
create view view_stu_dept
as
select s.id,s.name sname,s.deptno deptno1,d.deptno deptno2,d.name deptname
from student s,department d
where s.deptno = d.deptno;
#成功！
update view_stu_dept set sname = 'xiaoyi' where id = 1;
SQL> select * from view_stu_dept;
# 函数建视图
create view view_student_1 as select id,upper(name) sname from student;
SQL> select * from view_student_1;
ID SNAME
--------------------------------------- --------------------
1 XIAOYI
2 LI
3 ZHANG
select * from view_student_1 where sname = 'XIAOYI';
删除视图
drop view view_student_1;