中小企业建站可以怎么做wordpress主题 个人博客
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 C6 B5 A5 wa
链接视图
SQL> select * from student;SNO SNAME
----- -----1 B2 CSQL> select * from address;ID NAME
--------------------------------------- --------------------1 贵阳2 大连create view view_student_address
asselect s.sname,a.namefrom student s,address awhere 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 XIAOYI2 LI3 ZHANGselect * from view_student_1 where sname = 'XIAOYI';
删除视图
drop view view_student_1;
