如何将此代码从 oracle 转换为 redshift？

Question

我正在尝试在 redshift 中实现相同的功能，但我发现要做到这一点并不困难。由于 redshift 位于 postgresql 引擎的顶部，如果有人可以在 postgresql 中做到这一点，那将非常有帮助。基本上，代码在列级别获取前两个月的计数。如果没有准确的上个月计数，则为 0。

这是我的代码：

with abc(dateval,cnt) as(
    select 201908, 100 from dual union
    select 201907, 200 from dual union
    select 201906, 300 from dual union
    select 201904, 600 from dual)
select dateval, cnt, 
       last_value(cnt) over (order by dateval 
                             range between interval '1' month preceding 
                               and interval '1' month preceding ) m1,
       last_value(cnt) over (order by dateval 
                             range between interval '2' month preceding 
                               and interval '2' month preceding ) m2
  from (select to_date(dateval, 'yyyymm') dateval, cnt from abc)

我在 over by 子句中出现错误。我试图给演员（'1个月'作为间隔）但仍然失败。有人可以帮我解决这个 windows 功能吗？

预期输出：

问候

Answer 1

我会这样做。在 Redshift 中没有简单的方法来生成序列，我是否从任意表中选择 row_number() 来创建序列：

with abc(dateval,cnt) as(
    select 201908, 100 union
    select 201907, 200 union
    select 201906, 300 union
    select 201904, 600),
     cal(date) as (
         select
             add_months(
                 '20190101'::date,
                 row_number() over () - 1
            ) as date
         from <an arbitrary table to generate a sequence of rows> limit 10
     ),
     with_lag as (
         select
                dateval,
                cnt,
                lag(cnt, 1) over (order by date) as m1,
                lag(cnt, 2) over (order by date) as m2
         from abc right join cal on to_date(dateval, 'YYYYMM') = date
     )
select * from with_lag
where dateval is not null
order by dateval