如何避免在我的红移查询中使用限制子句？

Question

我有一个下面的查询，我使用limit 8 来获取日期值，如下所示：

SELECT (date_trunc('week', getdate() + INTERVAL '1 day')::date - 7 * (row_number() over (order by true) - 1) - INTERVAL '1 day')::date AS week_column
FROM dimensions.customer LIMIT 8

下面是我从上面的查询得到的输出：

2021-01-10
2021-01-03
2020-12-27
2020-12-20
2020-12-13
2020-12-06
2020-11-29
2020-11-22

有什么方法可以避免在上面的查询中使用limit 8 并且仍然得到相同的输出？我们的一个平台不允许我们运行查询，如果它有限制，所以想看看我是否可以在 sql redshift 中以不同的方式重写它？

以下是我使用dates CTE 的完整查询：

WITH dates AS (
    SELECT (date_trunc('week', getdate() + INTERVAL '1 day')::date - 7 * (row_number() over (order by true) - 1) - INTERVAL '1 day')::date AS week_column
    FROM dimensions.customer LIMIT 8
)
SELECT 
dates.week_column, 
'W' || ceiling(date_part('week', dates.week_column + INTERVAL '1 day')) AS week_number,
COUNT(DISTINCT features.client_id) AS total
FROM dimensions.program features 
JOIN dates ON features.last_update <= dates.week_column
WHERE features.type = 'capacity'
AND features.status = 'CURRENT'
GROUP BY dates.week_column
ORDER by dates.week_column DESC

我怎样才能以不同的方式重写我的dates CTE 查询，以便它可以给我相同的输出？

更新

如果我像这样运行查询，它会给我一个错误，但如果我在 from 内运行最里面的 sql 查询，那么它会给我数据。

SELECT *
FROM (SELECT (date_trunc('week', getdate() + INTERVAL '1 day')::date - 7 * (row_number() over (order by true) - 1) - INTERVAL '1 day')::date AS week_column,
              ROW_NUMBER() OVER () as seqnum
      FROM dimensions.customer
     ) c
WHERE seqnum <= 8;

错误是：

Invalid operation: Output timestamp out of range after subtracting constant. Details: ----------------------------------------------- error: Output timestamp out of range after subtracting constant. 

Answer 1

你可以使用窗口函数：

SELECT . . .   -- whatever columns you want
FROM (SELECT (date_trunc('week', getdate() + INTERVAL '1 day')::date - 7 * (row_number() over (order by true) - 1) - INTERVAL '1 day')::date AS week_column,
              ROW_NUMBER() OVER () as seqnum
      FROM dimensions.customer
     ) c
WHERE seqnum <= 8;

在 Redshift 中，ORDER BY 对于 ROW_NUMBER() 是可选的。一般来说，我会鼓励你有一个ORDER BY。您的查询也是如此。 LIMIT 通常与ORDER BY 一起使用。

编辑：

思路是一样的：

with dates as (
     ),
     q as (
      <your query here>
     )
select q.*
from (select q.*,
             row_number() over (order by weeks_column desc) as seqnum
      from q
     ) q
where seqnum <= 8;