Calculate moving average using SQL window functions with leading null's where not enough data is avaliable

Question

我想使用SQL窗口函数计算移动平均值。下面的2个“日”移动平均线示例基本上可以正常工作，但是如果只有一个数据点可用，它也可以计算平均线。我希望只要没有足够的数据，平均值就可以为空

create table average(
    nr int,
    value float
);

insert into average values (1, 2), (2, 4), (3, 6), (3, 8), (4, 10);

SELECT
    nr, 
    value, 
    AVG(value) OVER (ORDER BY nr ROWS BETWEEN 1 PRECEDING AND 0 FOLLOWING)::FLOAT AS "Moving-Average-2"
FROM average;

结果：

1   2   2
2   4   3
3   6   5
3   8   7
4   10  9

预期结果：

1   2   null
2   4   3
3   6   5
3   8   7
4   10  9

编辑1：当然，平均值可以不只是2。

Answer 1

您可以使用另一个窗口函数（COUNT()）来确保在进行计算之前窗口中至少有两条记录可用，例如：

SELECT
    nr, 
    value, 
    CASE WHEN COUNT(*) OVER(ORDER BY nr ROWS BETWEEN 1 PRECEDING AND 0 FOLLOWING) > 1
        THEN AVG(value) OVER (ORDER BY nr ROWS BETWEEN 1 PRECEDING AND 0 FOLLOWING)::FLOAT 
        ELSE NULL
    END AS "Moving-Average-2"
FROM average;

DB Fiddle 演示：

| nr  | value | Moving-Average-2 |
| --- | ----- | ---------------- |
| 1   | 2     |                  |
| 2   | 4     | 3                |
| 3   | 6     | 5                |
| 3   | 8     | 7                |
| 4   | 10    | 9                |