sql - PostgreSQL last_value 忽略空值

Question

我知道有人问过这个问题，但为什么下面的解决方案不起作用？我想用 idx 排序的最后一个非空值填充 value。

我看到的:

 idx | coalesce 
-----+----------
   1 |        2
   2 |        4
   3 |         
   4 |         
   5 |       10
(5 rows)

我想要的:

 idx | coalesce 
-----+----------
   1 |        2
   2 |        4
   3 |        4 
   4 |        4 
   5 |       10
(5 rows)

代码:

with base as (

    select 1    as idx
         , 2    as value

    union

    select 2    as idx
         , 4    as value

    union

    select 3    as idx
         , null as value

    union

    select 4    as idx
         , null as value

    union

    select 5    as idx
         , 10   as value
)

select idx
     , coalesce(value
              , last_value(value) over (order by case when value is null then -1
                                                 else idx
                                                 end))
from base
order by idx

Answer 1

你想要的是lag(ignore nulls)。这是使用两个窗口函数执行所需操作的一种方法。第一个定义 NULL 值的分组，第二个分配值:

select idx, value, coalesce(value, max(value) over (partition by grp))
from (select b.*, count(value) over (order by idx) as grp
      from base b
     ) b
order by idx;

您也可以通过使用数组在没有子查询的情况下执行此操作。基本上，取最后一个不计算 NULL 的元素:

select idx, value, 
       (array_remove(array_agg(value) over (order by idx), null))[count(value) over (order by idx)]
from base b
order by idx;

Here是一个数据库<> fiddle 。