Oracle視圖概述

使用視圖的原因：安全，簡化用戶的SQL語句，防止錯誤的發生，提高性能，讓數據更加有意義；

 

創建視圖示例：

CREATE VIEW emp_finAS SELECT hire_date,job_id,salary,commission_pct FROM employees;

 

 

當對兩張表進行關聯的時候通常用到兩種技術：nested loop,  hash join

nestedloop join uses an index to get to individual rows

hash joinreads the whole table into memory;

例

create view dept_emp as select /*+USE_HASH(employees departments)*/ department_name,last_name

fromdepartments natural join employees;

 

簡單視圖和複雜視圖

簡單視圖和複雜視圖的分類方法是：是否允許DML語句執行

簡單視圖允許DML語句：A simple view draws data from one detailtable, uses no functions ,and does no aggregation

複雜視圖不允許DML語句:A complexview can join detail table, use functions, and perform aggregations;

 

創建視圖

CREATE [OR REPLACE][FORCE | NOFORCE ] VIEW

viewname [(alias1)]

AS subquery

[WITH CHECK OPTION[CONSTRAINT constraintname]]

[WITH READ ONLY[CONSTRAINT constraintname]];

 

force：即使原表不存在也建立起一個視圖；

withcheck option:

with read only:組織通過視圖進行DML操作；

 

視圖必須通過成功編譯纔可以被使用；