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Oracle -- 视图与序列
2022-07-06 22:12:00 【攀登程序猿】
视图定义
是从一个或者多个数据表中导出的虚表(虚拟表),视图所对应的数据并不真正的存储在数据库中,比如将一个查询的结果定义为视图,则直接查询视图即可
优点:
(1)简化数据的操作:视图可以简化用户处理数据的方式
(2)着重与特定数据:不必要的数据或敏感数据可以不出现在视图中
(3)安全机制,不同的用户对数据访问权限不一样
(4)提供向后监控性,表改变,视图可以不变
视图操作
特点:
(1)视图不会实际存储,当视图允许修改时,修改视图中的数据,则表中的数据也被修改
(2)只读视图不允许修改数据
(3)带约束的视图
(4)多表关联也可以修改数据,基表的数据会跟随修改
create VIEW classstudent AS SELECT * FROM TBL_STUDENTS
// or replace是视图存在时,替换成新的,with check option是带检查约束的视图
create OR REPLACE VIEW classstudent AS SELECT * FROM TBL_STUDENTS WITH CHECK OPTION
// 创建只读视图
create OR REPLACE VIEW classstudent AS SELECT * FROM TBL_STUDENTS WITH READ ONLY
// 强制视图,即表不存在时创建的视图,用于动态生成表的场景
CREATE force VIEW view_test as SELECT * from tbl_test
// 复杂视图 多表查询,允许修改数据,会键保留表
CREATE VIEW view_student_class AS
SELECT * FROM TBL_STUDENTS JOIN TBL_CLASSES ON TBL_STUDENTS.STU_CID = TBL_CLASSES.CLASS_ID
// 聚合视图,聚合函数必须有列名
CREATE VIEW view_student as
SELECT COUNT(*) as count,STU_AGE as age FROM TBL_STUDENTS GROUP BY STU_AGE
注:
(1)键保留表,对于可更新的视图,在视图中的行和基表中的行之间必须具有一对一的关系,因为这样才能够反向映射到原基础表
(2)聚合函数做视图必须有列名
(3)聚合函数的视图无法修改
物化视图
物化视图是将查询的结果建立副本,占用存储空间,查询效率会高,但是会占用空间,未解决权限问题,后续学习。
更新方式:
COMPLETE 完全更新
FAST 增量更新,根据基表的更新日志,判断视图是否需要更新
ON COMMIT是在基表变化时刷新物化视图
ON COMMON是手动刷新
BUILD IMMEDIATE 创建视图时生成数据
ORA-01031: insufficient privileges
序列
创建一系列数字,mysql的主键自增是直接写的,oracle是需要创建序列的
--创建简单序列
CREATE sequence seq1;
--查询序列的下一个值,执行一次递增一个
SELECT seq1.NEXTval from dual;
--查询序列的当前值
select seq1.currval from dual;
CREATE sequence seq_withMax
maxvalue 20; --创建带有最大值的
SELECT seq_withMax.nextval FROM dual; -- 从一开始,每次递增1,最大到20
-- 跳跃的序列
CREATE sequence seq3
INCREMENT BY 2 -- 默认是1
START WITH 1000 --递增默认是MaxValue,递减是minvalue
maxvalue 1002
minvalue 100
cycle --是否循环 nocycle
cache 10; -- 每次放到内存中10个值,默认使用cache 20,不使用cache时配置为nocache,磁盘中保存到10了,缓存到20了,此时重启服务会从21开始,造成断号,有缓存必须小于一次的循环,即cache默认20*INCREMENT by = 200个,得比cache是10,小于200
select seq3.nextval from dual; -- 从1000开始,每次递增2,到1002后,重新从100开始
CREATE sequence seq4
INCREMENT BY -2
START WITH 1000
maxvalue 1001;
SELECT seq4.nextval FROM dual; -- 从1000开始,每次递减2
alter sequenct可以修改序列,不能修改start with
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