Oracle 开窗函数

在 Oracle 数据库中，开窗函数（Window Functions）是一类非常强大的 SQL 函数，用于在结果集的"窗口"上执行计算。这些窗口函数允许你在查询的每一行上执行聚合操作（如求和、平均、计数等），同时保留原始行的详细数据。

以下是一些常见的 Oracle 开窗函数及其用法：

1. ROW_NUMBER()

为结果集中的每一行分配一个唯一的序号。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank_in_dept 
FROM 
employees;

2. RANK()

类似于 ROW_NUMBER()，但如果有相同的值，它们会共享相同的排名，并且后续排名会跳过。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank_in_dept 
FROM 
employees;

3. DENSE_RANK()

类似于 RANK()，但后续排名不会跳过。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
DENSE_RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS dense_rank_in_dept 
FROM 
employees;

4. NTILE(n)

将结果集划分为 n 个桶，并为每一行分配一个桶号。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
NTILE(4) OVER (PARTITION BY department_id ORDER BY salary DESC) AS quartile 
FROM 
employees;

5. LAG() 和 LEAD()

用于访问结果集中当前行的前一行或后一行的数据。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
LAG(salary, 1) OVER (PARTITION BY department_id ORDER BY salary) AS prev_salary, 
LEAD(salary, 1) OVER (PARTITION BY department_id ORDER BY salary) AS next_salary 
FROM 
employees;

6. FIRST_VALUE() 和 LAST_VALUE()

返回窗口框架中的第一行或最后一行的值。

sql 复制代码

SELECT

employee_id,

department_id,

salary,

FIRST_VALUE(salary) OVER (PARTITION BY department_id ORDER BY salary RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS first_salary_in_dept,

LAST_VALUE(salary) OVER (PARTITION BY department_id ORDER BY salary RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS last_salary_in_dept

FROM

employees;

7. SUM(), AVG(), MIN(), MAX() 等聚合函数

在窗口上执行常见的聚合操作。

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
SUM(salary) OVER (PARTITION BY department_id ORDER BY salary RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cumulative_salary 
FROM 
employees;

窗口子句（Window Clause）

在开窗函数中，OVER 子句用于定义窗口的分区和排序。PARTITION BY 子句用于将结果集划分为分区，ORDER BY 子句用于定义每个分区内的排序顺序。

sql 复制代码

OVER ( 
[PARTITION BY partition_expression, ...] 
[ORDER BY sort_expression [ASC|DESC], ...] 
[ROWS|RANGE BETWEEN frame_start AND frame_end] 
)

PARTITION BY：将结果集划分为多个分区，每个分区独立计算。
ORDER BY：定义每个分区内的排序顺序。
ROWS 或 RANGE：定义窗口框架（frame），即窗口内包含的行范围。

示例：综合使用

sql 复制代码

SELECT 
employee_id, 
department_id, 
salary, 
ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank_in_dept, 
SUM(salary) OVER (PARTITION BY department_id ORDER BY salary RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cumulative_salary 
FROM 
employees;

以上示例展示了如何使用多个开窗函数，并结合 PARTITION BY 和 ORDER BY 子句进行复杂查询。