Question

我想编写一个从有序表中获取行的查询，同时聚合一列的值，直到所述聚合值达到所需的阈值。

另一个标准是通过阈值的违规行应该包含在查询结果中。

我已经在PostgreSQL中寻找其他解决方案，导致我创建以下查询：

SELECT * FROM (
    SELECT *, SUM(amount) OVER (ORDER BY amount DESC) AS running_amount
    FROM public.orders WHERE price = 0.09) AS t
WHERE t.running_amount <= 15;

此查询的问题是，它表示PostgreSQL窗口查询，如果给定行的列值不唯一，则会跳过所有行的列值聚合。

不幸的是，窗口查询不支持考虑不同值列的考虑。

我听到的一些替代方案仍然是通过创建PostgreSQL函数，虽然我不知道从哪里开始这种聚合查询。

如果有人有任何想法或技术诀窍，我将不胜感激。

Answer 1

将唯一列（主键）添加到窗口函数的ORDER BY子句中，例如：

SELECT * FROM (
    SELECT *, SUM(amount) OVER (ORDER BY amount DESC, id) AS running_amount
    FROM public.orders WHERE price = 0.09
) AS t
WHERE t.running_amount <= 15;

如果缺少唯一列，您可以使用系统列ctid.

您可以使用UNION ALL来获取超过阈值的违规行，例如：

WITH cte AS (
    SELECT *, SUM(amount) OVER (ORDER BY amount DESC, id) AS running_amount
    FROM public.orders 
    WHERE price = 0.09
)
SELECT * 
FROM cte
WHERE running_amount <= 15
UNION ALL (
    SELECT * 
    FROM cte
    WHERE running_amount > 15
    LIMIT 1
);

Answer 2

如果你想要最后一行（超过阈值的那一行），你有两个相对简单的选择。我的偏好是：

SELECT o.*
FROM (SELECT o.*,
             SUM(amount) OVER (ORDER BY amount DESC) AS running_amount
      FROM public.orders
      WHERE price = 0.09
     ) o
WHERE o.running_amount - o.amount <= 15;

替代方案是一个窗口条款：

SELECT o.*
FROM (SELECT o.*,
             SUM(amount) OVER (ORDER BY amount DESC
                               ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
                              ) AS running_amount
      FROM public.orders
      WHERE price = 0.09
     ) o
WHERE o.running_amount <= 15;

PostgreSQL：选择行，直到单个列的减少量超过阈值

2 个答案: