替换每个分区的 NULL 值

问题描述

我想在 NULL 列中为每个 device 填充 session_id 值，并使用关联的非 NULL 值。我怎样才能做到这一点？

以下是示例数据：

+------------+-------+---------+
| session_id | step  | device  |
+------------+-------+---------+
| 351acc     | step1 |         |
| 351acc     | step2 |         |
| 351acc     | step3 | mobile  |
| 351acc     | step4 | mobile  |
| 350bca     | step1 | desktop |
| 350bca     | step2 |         |
| 350bca     | step3 |         |
| 350bca     | step4 | desktop |
+------------+-------+---------+

期望的输出：

+------------+-------+---------+
| session_id | step  | device  |
+------------+-------+---------+
| 351acc     | step1 | mobile  |
| 351acc     | step2 | mobile  |
| 351acc     | step3 | mobile  |
| 351acc     | step4 | mobile  |
| 350bca     | step1 | desktop |
| 350bca     | step2 | desktop |
| 350bca     | step3 | desktop |
| 350bca     | step4 | desktop |
+------------+-------+---------+

解决方法

根据您的数据样本，每个会话有一个设备，因此您只需添加一个子查询即可从其他行中获取值

WITH j (session_id,step,device) AS (
  VALUES ('351acc','step1',NULL),('351acc','step2','step3','mobile'),'step4',('350bca','desktop'),'desktop')
) 
SELECT session_id,(SELECT DISTINCT device 
   FROM j q2
   WHERE q2.session_id = q1.session_id AND q2.device IS NOT NULL) AS device
FROM j q1 ORDER BY session_id,step;

 session_id | step  | device  
------------+-------+---------
 350bca     | step1 | desktop
 350bca     | step2 | desktop
 350bca     | step3 | desktop
 350bca     | step4 | desktop
 351acc     | step1 | mobile
 351acc     | step2 | mobile
 351acc     | step3 | mobile
 351acc     | step4 | mobile
(8 Zeilen)

演示：db<>fiddle

,

顺序正确的 window function first_value() 可能最便宜：

SELECT session_id,COALESCE(device,first_value(device) OVER (PARTITION BY session_id ORDER BY device IS NULL,step)
               ) AS device
FROM   tbl
ORDER  BY session_id DESC,step;

dbfiddle here

ORDER BY device IS NULL,step 最后对 NULL 值进行排序，因此选择最早的具有非空值的 step。见：

• Sorting null values after all others,except special

如果每个 session_id 的非空设备始终相同，您可以简化为仅 ORDER BY device IS NULL。而且你不需要COALESCE。

,

select session_id,coalesce(device,max(device) over (partition by session_id order by step desc)) device
from table