问题描述

我有一种情况，我试图从所有列中获取最后一个非空值.这是一些用于演示目的的数据.

I have a situation where I am trying to get last not null value from all the columns. here is some data to for demo puposes.

测试数据

DECLARE @t TABLE (ID INT, Col1 INT, Col2 INT, Col3 INT, Dated DATETIME)
INSERT INTO @t VALUES
(1, NULL, 100  , NULL, '20131210'),  --<-- Column2 from this row
(1, 20  , 200  , NULL, '20131209'),  --<-- Column1 from this row
(1, 30  , NULL , 300 , '20131208'),  --<-- Column3 from this row
(1, 40  , 400  , NULL, '20131207')

╔════╦══════╦══════╦══════╦═════════════════════════╗
║ ID ║ Col1 ║ Col2 ║ Col3 ║          Dated          ║
╠════╬══════╬══════╬══════╬═════════════════════════╣
║  1 ║ NULL ║ 100  ║ NULL ║ 2013-12-10 00:00:00.000 ║
║  1 ║ 20   ║ 200  ║ NULL ║ 2013-12-09 00:00:00.000 ║
║  1 ║ 30   ║ NULL ║ 300  ║ 2013-12-08 00:00:00.000 ║
║  1 ║ 40   ║ 400  ║ NULL ║ 2013-12-07 00:00:00.000 ║
╚════╩══════╩══════╩══════╩═════════════════════════╝

我已经建立了一个返回所需结果的查询.但想知道是否有更高效的方法来做到这一点.由于这种方式我每次都在查询整个表，我希望找到一种更有效的方法来获得这些结果.

I have built a query which returns the required results. But was wondering if there is a more performance efficient way of doing this. Since this way I am quering the whole table each time I hope to find a more efficient way of getting these results.

我的查询

SELECT ColumnName, Col1 AS Value, Dated 
 FROM
(
SELECT TOP 1 'Column1' AS ColumnName
            , Col1 
            , Dated
FROM @t
WHERE Col1 IS NOT NULL 
ORDER BY Dated DESC
)Q1

UNION ALL

SELECT * FROM
(
SELECT TOP 1 'Column2' AS ColumnName
            , Col2 
            , Dated
FROM @t
WHERE Col2 IS NOT NULL 
ORDER BY Dated DESC
)Q2

UNION ALL

SELECT * FROM
(
SELECT TOP 1 'Column3' AS ColumnName
            , Col3 
            , Dated
FROM @t
WHERE Col3 IS NOT NULL 
ORDER BY Dated DESC
)Q3

结果集

╔════════════╦═══════╦═════════════════════════╗
║ ColumnName ║ Value ║          Dated          ║
╠════════════╬═══════╬═════════════════════════╣
║ Column1    ║    20 ║ 2013-12-09 00:00:00.000 ║
║ Column2    ║   100 ║ 2013-12-10 00:00:00.000 ║
║ Column3    ║   300 ║ 2013-12-08 00:00:00.000 ║
╚════════════╩═══════╩═════════════════════════╝

它返回正确的结果.但我确信可以通过 更简单/高效的查询 来完成.任何帮助或指针都非常受欢迎

It returns the correct results. but I am sure it can be done with a bit more simpler/efficient query . Any help or pointer is much appericiated

推荐答案

非常简单UNPIVOT 操作和适当的 ROW_NUMBER 选择最近的值.UNPIVOT 自动消除 NULL 值:

Pretty much a straightforward UNPIVOT operation and an appropriate ROW_NUMBER to select the most recent value. The UNPIVOT automatically eliminates NULL values:

;With Numbered as (
     select *,ROW_NUMBER() OVER (PARTITION BY Col ORDER BY Dated desc) rn
     from @t
     unpivot (Value for Col in (Col1,Col2,Col3)) p
)
select * from Numbered where rn = 1

结果:

ID          Dated                   Value       Col      rn
----------- ----------------------- ----------- -------- --------------------
1           2013-12-09 00:00:00.000 20          Col1     1
1           2013-12-10 00:00:00.000 100         Col2     1
1           2013-12-08 00:00:00.000 300         Col3     1

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