用每个组中的先前非空值替换空值

Question

我正在通过自定义 SQL 查询连接到 Microsoft SQL Server on Tableau。我有一个包含 3 个字段 DateTime、TagName、Value 的 table，我想用每组 TagName 中的最后一个（关于 DateTime 值）非空值替换值字段中的空值。

|---------------------|------------------|-----------------|
|     DateTime        |     TagName      |      Value
|---------------------|------------------|-----------------
|  15.04.2019 16:51:30|         A        |       10
|---------------------|------------------|----------------- 
|  15.04.2019 16:52:42|         A        |       NULL
|---------------------|------------------|----------------- 
|  15.04.2019 16:53:14|         A        |       NULL
|---------------------|------------------|----------------- 
|  15.04.2019 17:52:14|         A        |       15
|---------------------|------------------|----------------- 
|  15.04.2019 16:51:30|         B        |       NULL
|---------------------|------------------|----------------- 
|  15.04.2019 16:52:42|         B        |       NULL
|---------------------|------------------|-----------------
|  15.04.2019 16:53:14|         B        |       NULL
|---------------------|------------------|----------------- 
|  15.04.2019 17:52:14|         B        |       15
|---------------------|------------------|-----------------|

新的 table 应该是这样的：

|---------------------|------------------|-----------------|
|     DateTime        |     Computer     |      Value
|---------------------|------------------|-----------------
|  15.04.2019 16:51:30|         A        |       10
|---------------------|------------------|----------------- 
|  15.04.2019 16:52:42|         A        |       10
|---------------------|------------------|----------------- 
|  15.04.2019 16:53:14|         A        |       10
|---------------------|------------------|----------------- 
|  15.04.2019 17:52:14|         A        |       15
|---------------------|------------------|----------------- 
|  15.04.2019 16:51:30|         B        |       0
|---------------------|------------------|----------------- 
|  15.04.2019 16:52:42|         B        |       0
|---------------------|------------------|-----------------
|  15.04.2019 16:53:14|         B        |       0
|---------------------|------------------|----------------- 
|  15.04.2019 17:52:14|         B        |       15
|---------------------|------------------|-----------------|

这已经是我尝试过的方法，但它替换了 NULL 值而不考虑 TagNames 值（在本例中只有一个 TagName）。

SELECT  Computer, DateTime
,       CASE 
        WHEN Value IS NULL 
        THEN                                
       (SELECT TOP 1 Value 
        FROM History 
        WHERE DateTime<T.DateTime 
              AND TagName='RM02EL00CPT81.rEp'
              AND DateTime >='2018-12-31 23:59:00' 
              AND wwRetrievalMode='Delta'
              AND Value IS NOT NULL ORDER BY DateTime DESC
       ) 
        ELSE Value 
        END 
        AS ValueNEW
FROM History T
WHERE  TagName='RM02EL00CPT81.rEp' AND DateTime >='2018-12-31 23:59:00' AND wwRetrievalMode='Delta'

我想通过添加 OVER(PARTITION BY TagName) 来做几乎相同的事情，但它抛出了一个错误。（这是因为它不适用于 SELECT TOP 1。）

Answer 1

试试这个

;WITH CTE([DateTime],TagName,Valu)
AS
(
SELECT '15.04.2019 16:51:30','A' , 10    UNION ALL
SELECT '15.04.2019 16:52:42','A' , NULL  UNION ALL
SELECT '15.04.2019 16:53:14','A' , NULL  UNION ALL
SELECT '15.04.2019 17:52:14','A' , 15    UNION ALL
SELECT '15.04.2019 16:51:30','B' , NULL  UNION ALL
SELECT '15.04.2019 16:52:42','B' , NULL  UNION ALL
SELECT '15.04.2019 16:53:14','B' , NULL  UNION ALL
SELECT '15.04.2019 17:52:14','B' , 15
)
SELECT [DateTime],TagName As Computer,
        ISNULL(CASE WHEN Valu IS NOT NULL   
            THEN Valu
            ELSE 
                (
                SELECT TOP 1 Valu FROM  
                CTE i
                WHERE i.TagName = o.TagName     
                ) END,0) As Valu
FROM CTE o

结果

DateTime                Computer    Valu
---------------------------------------------
15.04.2019 16:51:30     A           10
15.04.2019 16:52:42     A           10
15.04.2019 16:53:14     A           10
15.04.2019 17:52:14     A           15
15.04.2019 16:51:30     B           0
15.04.2019 16:52:42     B           0
15.04.2019 16:53:14     B           0
15.04.2019 17:52:14     B           15

Answer 2

这是一个 "classic" 差距和孤岛问题。您可以在不进行 2 次扫描或使用 window 函数进行三角连接的情况下实现此目的：

WITH VTE AS(
    SELECT CONVERT(datetime, [DateTime],104) AS [DateTime],
           TagName,
           [Value]
    FROM (VALUES ('15.04.2019 16:51:30','A',10  ),
                 ('15.04.2019 16:52:42','A',NULL),
                 ('15.04.2019 16:53:14','A',NULL),
                 ('15.04.2019 17:52:14','A',15  ),
                 ('15.04.2019 16:51:30','B',NULL),
                 ('15.04.2019 16:52:42','B',NULL),
                 ('15.04.2019 16:53:14','B',NULL),
                 ('15.04.2019 17:52:14','B',15  )) V([DateTime],TagName,[Value])),
Grps AS(
    SELECT [DateTime],
           TagName,
           [Value],
           COUNT(CASE WHEN [Value] IS NOT NULL THEN 1 END) OVER (PARTITION BY TagName ORDER BY [DateTime]
                                                                 ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS Grp
    FROM VTE)
SELECT DateTime,
       TagName,
       ISNULL(MAX([Value]) OVER (PARTITION BY TagName, Grp),0) AS [Value]
FROM Grps
ORDER BY TagName, [DateTime]

Answer 3

所以您正在尝试从 Wonderware Historian 检索数据。也许您不需要任何窗口化和替换，因为 Historian 检索引擎应该能够为您提供您需要的数据，并且没有空值。试试这个：

select DateTime, TagName as Computer, Value
from History
where TagName in ('A', 'B') --put here the tagnames you want to retrieve
and DateTime > '2018-12-31'
AND wwRetrievalMode='Delta'
order by TagName, DateTime

用每个组中的先前非空值替换空值

Replacing null values with the previous non-null value in each group

sql-server

null

wonderware

gaps-and-islands