如何用前一行的值填充空列？

Posted 2023-04-12

【中文标题】如何用前一行的值填充空列？

【英文标题】：How do I fill null columns with values from the previous row?

【发布时间】：2019-11-16 23:02:23

【问题描述】：

我正在尝试在这里执行一些操作。我想让所有的列都充满值。但是当我有空列时，我想用前一个非空列中的值填充它。

with cte as (
select '2019-11-12 16:01:55' as timestamp, null as owner_id, null as owner_assigneddate, null as lastmodifieddate union all
select '2019-11-12 19:03:18' as timestamp, 39530934 as owner_id, '2019-11-12 19:03:18' as owner_assigneddate, '2019-11-12 19:03:18' as lastmodifieddate union all
select '2019-11-12 19:03:19' as timestamp, null as owner_id, null as owner_assigneddate, '2019-11-12 19:03:19' as lastmodifieddate union all
select '2019-11-12 19:03:20' as timestamp, null as owner_id, null as owner_assigneddate, '2019-11-12 19:03:20' as lastmodifieddate union all
select '2019-11-12 19:03:31' as timestamp, 40320368 as owner_id, '2019-11-12 19:03:31' as owner_assigneddate, '2019-11-12 19:03:31' as lastmodifieddate union all
select '2019-11-12 19:03:33' as timestamp, null as owner_id, null as owner_assigneddate, '2019-11-12 19:03:33' as lastmodifieddate union all
select '2019-11-12 19:03:56' as timestamp, null as owner_id, null as owner_assigneddate, '2019-11-12 19:03:356' as lastmodifieddate)

select timestamp,
       owner_id,
       owner_assigneddate,
       lastmodifieddate,
       COALESCE(owner_id, LEAD(owner_id) OVER(ORDER BY timestamp DESC)) AS test_column
from cte order by timestamp asc 

通过上一个查询，我已经设法将值仅放在下一行。

我想要做的是让所有列都填充基于前一行的值。 第 4 行的值应为 39530934，第 7 行的值应为 40320368。 我想我在这里遗漏了一些东西，但我不知道是什么。

【参考方案1】：

在 BigQuery 中使用 LAST_VALUE() 和 IGNORE NULLS 选项和 COALESCE()：

select timestamp,
       COALESCE(owner_id, last_value(owner_id ignore nulls) over (order by timestamp)) as owner_id,
       COALESCE(owner_assigneddate, LAST_VALUE(owner_assigneddate IGNORE NULLS) OVER (ORDER BY TIMESTAMP)) as owner_assigneddate,
       COALESCE(lastmodifieddate, LAST_VALUE(lastmodifieddate IGNORE NULLS) OVER (ORDER BY TIMESTAMP)) as lastmodifieddate
from cte order by timestamp asc 

【参考方案2】：

就问题而言，Big Query 不支持窗口函数中的ignore null。这是一个解决方案，它依赖于窗口最大值来定位保存最后一个非空owner_id 的记录（这假设时间戳的唯一性）。有了这些信息，您就可以引入相应的 owner_id 并加入。

select 
    c.timestamp,
    coalesce(c.owner_id, c_lag.owner_id) owner_id,
    c.owner_assigneddate,
    c.lastmodifieddate
from 
    (
        select
            cte.*,
            max(case when owner_id is not null then timestamp end) 
                over(order by timestamp rows unbounded preceding) target_timestamp
        from cte
    ) c
    left join cte c_lag 
        on c.owner_id is null 
        and c_lag.timestamp = c.target_timestamp

Demo on DB Fiddle：

时间戳 |所有者 ID |所有者分配日期 |最后修改日期 :----------------- | --------: | :----------------- | :-------------------- 2019-11-12 16:01:55 | 空 | 空 | 空 2019-11-12 19:03:18 | 39530934 | 2019-11-12 19:03:18 | 2019-11-12 19:03:18 2019-11-12 19:03:19 | 39530934 | 空 | 2019-11-12 19:03:19 2019-11-12 19:03:20 | 39530934 | 空 | 2019-11-12 19:03:20 2019-11-12 19:03:31 | 40320368 | 2019-11-12 19:03:31 | 2019-11-12 19:03:31 2019-11-12 19:03:33 | 40320368 | 空 | 2019-11-12 19:03:33 2019-11-12 19:03:56 | 40320368 | 空 | 2019-11-12 19:03:356

注意：如果需要，为了更好地理解逻辑，您可以独立运行内部查询以查看它返回的内容（请参阅 db fiddle）。

---

编辑

重读本文，我发现window max提供的信息已经在您的原始数据中，在owner_assigneddate列中......所以这要简单得多：

select 
    c.timestamp,
    coalesce(c.owner_id, c_lag.owner_id) owner_id,
    c.owner_assigneddate,
    c.lastmodifieddate
from
    cte c
    left join cte c_lag 
        on c.owner_id is null 
        and c_lag.timestamp = c.owner_assigneddate

【参考方案3】：

这应该适用于您的 cte 定义：

...
select timestamp,
       owner_id,
       owner_assigneddate,
       lastmodifieddate,
       LAST_VALUE(owner_id IGNORE NULLS) 
            OVER(ORDER BY timestamp ASC ROWS BETWEEN 
                 UNBOUNDED PRECEDING AND CURRENT ROW) AS test_column
from cte order by timestamp asc