Get Last NOT NULL values from all the columns

问题

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.

Test Data

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

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.

My Query

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

Result Set

╔════════════╦═══════╦═════════════════════════╗
║ 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 ║
╚════════════╩═══════╩═════════════════════════╝

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

回答1:

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

Result:

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

回答2:

I think the in vertical row format your UNION option is better, but if you need it in horizontal column format then you can use self join like below

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

来源：https://stackoverflow.com/questions/21042284/get-last-not-null-values-from-all-the-columns