Oracle视图基础

1. 基本语法

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CREATE [OR REPLACE] [FORCE | NOFORCE] VIEW view_name
[(column_name [, column_name]...)]
AS subquery
[WITH {CHECK OPTION | READ ONLY} CONSTRAINT constraint_name];

2. 简单视图示例

创建基本视图

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-- 创建员工视图
CREATE VIEW emp_view AS
SELECT employee_id, first_name, last_name, salary, department_id
FROM employees
WHERE department_id = 20;

创建带别名的视图

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CREATE VIEW emp_dept_view (emp_id, name, dept_name, salary) AS
SELECT e.employee_id, 
       e.first_name || ' ' || e.last_name,
       d.department_name,
       e.salary
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

只读视图

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CREATE OR REPLACE VIEW read_only_emp_view AS
SELECT employee_id, first_name, last_name, hire_date
FROM employees
WITH READ ONLY CONSTRAINT ro_emp_view;

带检查选项的视图

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CREATE OR REPLACE VIEW high_salary_emp AS
SELECT employee_id, first_name, salary
FROM employees
WHERE salary > 5000
WITH CHECK OPTION CONSTRAINT chk_salary;

5. 物化视图

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-- 创建物化视图
CREATE MATERIALIZED VIEW mv_emp_summary
REFRESH COMPLETE ON DEMAND
AS
SELECT department_id,
       COUNT(*) emp_count,
       AVG(salary) avg_salary
FROM employees
GROUP BY department_id;

6. 使用CTE（公共表表达式）创建复杂视图

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CREATE OR REPLACE VIEW complex_analysis_view AS
WITH 
-- 第一步：基础数据处理
emp_data AS (
    SELECT e.employee_id,
           e.first_name || ' ' || e.last_name AS full_name,
           e.salary,
           e.department_id,
           e.hire_date,
           EXTRACT(YEAR FROM SYSDATE) - EXTRACT(YEAR FROM e.hire_date) AS years_of_service
    FROM employees e
    WHERE e.salary > 3000
),

-- 第二步：部门汇总
dept_summary AS (
    SELECT d.department_id,
           d.department_name,
           COUNT(ed.employee_id) AS emp_count,
           AVG(ed.salary) AS avg_salary
    FROM departments d
    LEFT JOIN emp_data ed ON d.department_id = ed.department_id
    GROUP BY d.department_id, d.department_name
),

-- 第三步：薪资等级分类
salary_analysis AS (
    SELECT ed.department_id,
           SUM(CASE WHEN ed.salary < 5000 THEN 1 ELSE 0 END) AS low_salary_count,
           SUM(CASE WHEN ed.salary BETWEEN 5000 AND 10000 THEN 1 ELSE 0 END) AS medium_salary_count,
           SUM(CASE WHEN ed.salary > 10000 THEN 1 ELSE 0 END) AS high_salary_count
    FROM emp_data ed
    GROUP BY ed.department_id
)

-- 最终结果
SELECT ds.department_name,
       ds.emp_count,
       ds.avg_salary,
       sa.low_salary_count,
       sa.medium_salary_count,
       sa.high_salary_count,
       ROUND(ds.avg_salary * ds.emp_count) AS total_dept_salary
FROM dept_summary ds
LEFT JOIN salary_analysis sa ON ds.department_id = sa.department_id
ORDER BY ds.avg_salary DESC;

7. 视图操作

修改视图

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-- 替换现有视图
CREATE OR REPLACE VIEW emp_view AS
SELECT employee_id, first_name, last_name, salary, hire_date
FROM employees
WHERE salary > 3000;

删除视图

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DROP VIEW emp_view;

重新编译视图

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ALTER VIEW emp_view COMPILE;