窗口函数first_value的异常行为 [英] Unexpected behavior of window function first_value

问题描述

我有2列-订单号，值。表值构造函数：

I have 2 columns - order no, value. Table value constructor:

(1, null)
,(2, 5)
,(3, null)
,(4, null)
,(5, 2)
,(6, 1)

我需要获取

(1, 5) -- i.e. first nonnull Value if I go from current row and order by OrderNo
,(2, 5)
,(3, 2) -- i.e. first nonnull Value if I go from current row and order by OrderNo
,(4, 2) -- analogous
,(5, 2)
,(6, 1)

这是我认为应该可以使用的查询。

This is query that I think should work.

;with SourceTable as (
    select *
        from (values
            (1, null)
            ,(2, 5)
            ,(3, null)
            ,(4, null)
            ,(5, 2)
            ,(6, 1)
        ) as T(OrderNo, Value)
)
select
       *
       ,first_value(Value) over (
           order by
               case when Value is not null then 0 else 1 end
               , OrderNo
           rows between current row and unbounded following
       ) as X
   from SourceTable
order by OrderNo

问题在于它返回与SourceTable完全相同的结果集。我不明白为什么。例如，如果处理了第一行（OrderNo = 1），我希望X列返回5，因为框架应该包含所有行（当前行和未绑定的行），并且它按Value排序-首先是非null，然后是OrderNo。因此，框架中的第一行应为OrderNo = 2。显然，它不是那样工作的，但我不明白为什么。

The issue is that it returns exactly same resultset as SourceTable. I don't understand why. E.g., if first row is processed (OrderNo = 1) I'd expect column X returns 5 because frame should include all rows (current row and unbound following) and it orders by Value - nonnulls first, then by OrderNo. So first row in frame should be OrderNo=2. Obviously it doesn't work like that but I don't get why.

如果有人解释了第一帧的结构，我将不胜感激。我需要SQL Server和Postgresql。

Much appreciated if someone explains how is constructed the first frame. I need this for SQL Server and also Postgresql.

非常感谢

推荐答案

很容易看出为什么如果按的情况对结果进行排序（如果Value不为null则为0，否则1结束，orderno

It's pretty easy to see why first_value doesn't work if you order the results by case when Value is not null then 0 else 1 end, orderno

 orderno | value | x
---------+-------+---
       2 |     5 | 5
       5 |     2 | 2
       6 |     1 | 1
       1 |       |
       3 |       |
       4 |       |
(6 rows)

对于orderno = 1，框架中没有任何内容

For orderno=1, there's nothing after it in the frame that would be not-null.

相反，我们可以使用count作为子查询的窗口函数将订单分为几组。然后，我们使用max作为该组上的窗口函数（这是任意的，min也可以工作）来获得该组中的一个非空值：

Instead, we can arrange the orders into groups using count as a window function in a sub-query. We then use max as a window function over that group (this is arbitrary, min would work just as well) to get the one non-null value in that group:

with SourceTable as (
    select *
        from (values
            (1, null)
            ,(2, 5)
            ,(3, null)
            ,(4, null)
            ,(5, 2)
            ,(6, 1)
        ) as T(OrderNo, Value)
)
select orderno, order_group, max(value) OVER (PARTITION BY order_group) FROM (

    SELECT *,
       count(value) OVER (ORDER BY orderno DESC) as order_group
   from SourceTable
   ) as sub
order by orderno;
 orderno | order_group | max
---------+-------------+-----
       1 |           3 |   5
       2 |           3 |   5
       3 |           2 |   2
       4 |           2 |   2
       5 |           2 |   2
       6 |           1 |   1
(6 rows)

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