Use previous state of data

All we need is an easy explanation of the problem, so here it is.

I am trying to carry forward the previous state of data wherever data is not available. For example, I have following data;

Month Order Amount
1 Item1 100
1 Item2 200
1 Item3 300
2 NULL NULL
3 Item4 100
4 NULL NULL
5 NULL NULL
6 Item1 100

So in the above example I want to use 1st month’s information to be carry forwarded as it is for 2nd month. Also since 3rd month’s information is available then use it for the month 4 and 5. Hence the output will be following:

Month Order Amount
1 Item1 100
1 Item2 200
1 Item3 300
2 Item1 100
2 Item2 200
2 Item3 300
3 Item4 100
4 Item4 100
5 Item4 100
6 Item1 100

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

WITH 
cte AS ( SELECT DISTINCT 
                Month, 
                MAX(CASE WHEN [Order] IS NOT NULL 
                         THEN Month 
                         END) OVER (ORDER BY Month 
                                    ROWS BETWEEN UNBOUNDED PRECEDING 
                                             AND 1 PRECEDING) prev
         FROM test )
SELECT Month, [Order], Amount
FROM test
WHERE [Order] IS NOT NULL
UNION ALL
SELECT t1.Month, t2.[Order], t2.Amount
FROM test t1
JOIN cte ON t1.Month = cte.Month
JOIN test t2 ON t2.Month = cte.prev
WHERE t1.[Order] IS NULL
ORDER BY Month, [Order];

fiddle with some explanations

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

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