T-SQL query update null values - sql-server

Question

I have a very specific problem in T-SQL.
If I can solve this example case I give you I think I will be able to solve my original case.
Having this data in a table:
DECLARE #Test TABLE
(
Value INT
,Date DATETIME2(7)
);
INSERT INTO #Test
VALUES
(NULL, '2011-01-01 10:00'),
(NULL, '2011-01-01 11:00'),
(2, '2011-01-01 12:00'),
(NULL, '2011-01-01 13:00'),
(3, '2011-01-01 14:00'),
(NULL, '2011-01-01 15:00'),
(NULL, '2011-01-01 16:00'),
(4, '2011-01-01 17:00'),
(NULL, '2011-01-01 18:00'),
(5, '2011-01-01 19:00'),
(6, '2011-01-01 20:00')
I need to select this output:
Value Date
2 2011-01-01 10:00
2 2011-01-01 11:00
2 2011-01-01 12:00
2 2011-01-01 13:00
3 2011-01-01 14:00
3 2011-01-01 15:00
3 2011-01-01 16:00
4 2011-01-01 17:00
4 2011-01-01 18:00
5 2011-01-01 19:00
6 2011-01-01 20:00
To give some explanation. If value is NULL somewhere I need to update with the value from the previous hour. If there are several null values in a row the closest earlier hour with a non null value propagates and fills all these null values. Also if the first hour of the day is null then the earliest hour on the day with a non null value propagates downwards like 2 in this case. In your case you can assume that at least one value is non null value.
My ambition is to solve this with Common table expressions or something similar. With the cursor way I think I would have the solution in short bit of time if I try but my attempts with CTEs and recursive CTEs have failed so far.

Answer 1

Since your condition is not always the same this is a little bit more difficult. In your example, the first two rows need to get their values from the first value with a later date, in the other cases they need to get the values from previous dates. If you would always need to look previous dates, you could simple do this query:
SELECT B.Value,
A.[Date]
FROM #Test A
OUTER APPLY (SELECT TOP 1 *
FROM #Test
WHERE [Date] <= A.[Date] AND Value IS NOT NULL
ORDER BY [Date] DESC) B
But in your case, I think that you need this instead:
SELECT ISNULL(B.Value,C.Value) Value,
A.[Date]
FROM #Test A
OUTER APPLY (SELECT TOP 1 *
FROM #Test
WHERE [Date] <= A.[Date] AND Value IS NOT NULL
ORDER BY [Date] DESC) B
OUTER APPLY (SELECT TOP 1 *
FROM #Test
WHERE Value IS NOT NULL
ORDER BY [Date]) C

Answer 2

try this:
select
t.value, t.date
,COALESCE(t.value
,(select MAX(tt.value) from #Test tt WHERE t.Date>tt.Date)
,(SELECT MIN(ttt.Value) FROM #Test ttt Where ttt.Date IS NOT NULL)
) AS UseValue
from #Test t
OUTPUT:
value date UseValue
----------- ----------------------- -----------
NULL 2011-01-01 10:00:00.000 2
NULL 2011-01-01 11:00:00.000 2
2 2011-01-01 12:00:00.000 2
NULL 2011-01-01 13:00:00.000 2
3 2011-01-01 14:00:00.000 3
NULL 2011-01-01 15:00:00.000 3
NULL 2011-01-01 16:00:00.000 3
4 2011-01-01 17:00:00.000 4
NULL 2011-01-01 18:00:00.000 4
5 2011-01-01 19:00:00.000 5
6 2011-01-01 20:00:00.000 6