Put not null values from above rows to current rows having null values .

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CustomerID FirstName CreateDate
1 Jonah 2011-09-01 00:00:00.000
2 Jonathan NULL
3 Leno 2014-05-05 00:00:00.000
4 James NULL
5 Tom 2018-05-01 00:00:00.000
6 Harry NULL
7 Ricky NULL
8 Katheriene NULL

Problem : Replace null values on date column from not null value of above.
I am use case statement to get below result but its very slow.
Expected output:
CreateDate CreateDate
2011-09-01 00:00:00.000 2011-09-01 00:00:00.000
NULL 2011-09-01 00:00:00.000
2014-05-05 00:00:00.000 2014-05-05 00:00:00.000
NULL 2014-05-05 00:00:00.000
2018-05-01 00:00:00.000 2018-05-01 00:00:00.000
NULL 2018-05-01 00:00:00.000
NULL 2018-05-01 00:00:00.000
NULL 2018-05-01 00:00:00.000

What I have tried:

SELECT main.CreateDate,CASE WHEN CreateDate IS NULL
THEN (SELECT TOP 1 sub.CreateDate FROM dbo.Customer sub WHERE sub.CustomerID <= main.CustomerID
order by sub.CreateDate desc )
ELSE main.CreateDate
END AS CreateDate
FROM dbo.Customer main;

Posted 21-Jul-18 21:30pm

jaykeymailbox

Updated 23-Jul-18 7:05am

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Richard Deeming · Answer 1 · 2018-07-23T07:06:00

Assuming SQL 2012 or higher, this technique from Dwain Camps might help:

SQL

WITH cteCount As
(
    SELECT
        CustomerID,
        FirstName,
        CreateDate,
        COUNT(CreateDate) OVER (ORDER BY CustomerID) As C
    FROM
        dbo.Customer
),
cteRN As
(
    SELECT
        CustomerID,
        FirstName,
        CreateDate,
        ROW_NUMBER() OVER (PARTITION BY C ORDER BY CustomerID) - 1 As RN
    FROM
        cteCount
)
SELECT
    CustomerID,
    FirstName,
    CreateDate,
    CASE
        WHEN CreateDate Is Not Null THEN CreateDate
        ELSE LAG(CreateDate, RN) OVER (ORDER BY CustomerID, CreateDate)
    END As CreateDate
FROM
    cteRN
ORDER BY
    CustomerID
;

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