Question

我有一种情况，我试图从所有列中获取最后一个非空值。这里有一些数据用于演示目的。

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 ║
╚════╩══════╩══════╩══════╩═════════════════════════╝

我已经构建了一个返回所需结果的查询。但是想知道是否有更高性能的方式来做到这一点。从这个方面来说，每当我希望找到一种更有效的方法来获得这些结果时，我就会查询整个表格。

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 ║
╚════════════╩═══════╩═════════════════════════╝

返回正确结果。但我相信它可以通过一些更简单/更有效的查询来完成。任何帮助或指针都很明显

Answer 1

几乎是一个简单的UNPIVOT操作和一个适当的ROW_NUMBER来选择最新的值。 UNPIVOT会自动删除NULL值：

;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

Answer 2

我认为你的UNION选项的垂直行格式更好，但如果你需要水平列格式，那么你可以使用下面的自连接

SELECT top 1 t1.Col1, t2.Col2, t3.col3
FROM @t t1
inner join @t t2 on t2.col2 is not null
inner join @t t3 on t3.col3 is not null
where t1.col1 is not null
order by t1.dated desc

从所有列获取Last NOT NULL值

2 个答案: