sql - Netezza 中的有界累积和

Question

我知道如何在其基本公式中使用累积总和，代码如下:

Table Name: Employees
dept_id salary
-------------
10     1000
10     1000
10     2000
10     3000
20     5000
20     6000
20     NULL

SELECT dept_id,
       salary,
       SUM(salary) OVER(PARTITION BY dept_id 
             ORDER BY salary ASC 
             rows unbounded preceding) cum_sum
FROM   Employees;

dept_id   salary  cum_sum  
--------------------------
10        1000    1000     
10        1000    2000     
10        2000    4000     
10        3000    7000     
20        5000    5000     
20        6000    11000    
20        NULL    11000    

但是如何将累积总和限制为仅前 N 行？
例如，将累积总和限制为当前行和前两行。

dept_id   salary  cum_sum  
--------------------------
10        1000    1000     
10        1000    2000     
10        2000    4000     
10        3000    6000     
20        5000    5000     
20        6000    11000    
20        NULL    11000    

Answer 1

SQL 语法是:

SELECT dept_id,
       salary,
       SUM(salary) OVER(PARTITION BY dept_id 
             ORDER BY salary ASC 
             rows between <N> preceding and current row) cum_sum
FROM   Employees;