iterative loop to select row - sql

Question

I need to be able to SELECT a specific row because SET only holds one row.
On line 5, I have to declare #json so that OPENJSON can work.
And that's why i need the RowNo at this point in the query. However, at this point, RowNo is not defined.
How would i go about fixing this problem? It seems that it doesn't matter how i spin it, something is always gonna be undefined. How would i point to a specific row on line 5 as desired?
DECLARE #loopCounter INT = 0;
WHILE #loopCounter < (SELECT count(Id) FROM ProcessEventMessages)
BEGIN
PRINT 'Changing...';
DECLARE #json NVARCHAR(MAX) = (SELECT Data FROM ProcessEventMessages WHERE RowNo = #loopCounter) -- here
SELECT * FROM OPENJSON(#json)
WITH (
message varchar(200) '$.message',
machineId varchar(200) '$.machineId',
machineName int '$.machineName',
ipAddress varchar(200) '$.ipAddress',
LocalTime datetime2(7) '$.time'
) AS ChangeTime;
;WITH CTE AS(
SELECT *,ROW_NUMBER() OVER(Order by Id ASC) AS RowNo
FROM ProcessEventMessages
)
UPDATE CTE
SET Data = JSON_MODIFY(#json,'$.time', FORMAT(DATEADD(hour,-2,JSON_VALUE(#json,'$.time')),'yyyy-MM-ddTHH:mm:ss.fff'))
WHERE RowNo = #loopCounter
SET #loopCounter = #loopCounter + 1;
END;
PRINT 'Done';
GO

Answer 1

You can use CROSS APPLY instead of a loop. Try:
WITH CTE AS (
SELECT
LocalTime
, Data
FROM dbo.ProcessEventMessages
CROSS APPLY OPENJSON(Data)
WITH (
LocalTime datetime2(7) '$.time'
) AS ChangeTime
)
UPDATE CTE
SET Data = JSON_MODIFY(Data,'$.time', FORMAT(DATEADD(hour,-2,LocalTime),'yyyy-MM-ddTHH:mm:ss.fff'));