Need to set start date to rowNum - 1

I have a stored procedure that I need to check the first row of data and if column1(coolHours) is null then I need to set the startDate to the same time as column2(completed) in the previous row.

SO in the image below I need to set startDate in the query to the completed value of row10 (rowNum-1)
 

The query that I am using is here - I have tried several different ideas and none have worked so far!
    BEGIN
      DECLARE startDate VARCHAR (50);
      DECLARE endDate   VARCHAR (50);
      DECLARE mylogID   INT;
      DECLARE myItemId varchar (50);
      DECLARE myItemId2 varchar (50);
      DECLARE xHours varchar (50);
    
      SET startDate = FROM_UNIXTIME(fromDate/1000);
      SET endDate   = FROM_UNIXTIME(toDate/1000);
      SET mylogID   = (SELECT logID FROM logs WHERE details LIKE 'FCT%' LIMIT 1);
      SET myItemID = (SELECT i.itemId FROM items i  WHERE i.name LIKE '%KW PRE%');
      SET myItemID2 = (SELECT i.itemId FROM items i  WHERE  i.name LIKE '%KW STA%');
    
      CALL reportCreateFreeCoolingTempTable(fromDate,toDate,'UTC');
         
      SET @sql = NULL;
      SET @sql = CONCAT(
      'SELECT 
        rowNum,l.logId,l.completed,l.next_completed,isError,charge,totalTimeRange,startingDate,endingDate,coolingHours,
        databaseName,i.name,l.itemId, kwDifference, cost, costT,l.details,timeInSeconds
      FROM tempTable3 l
       inner join items i ON l.logId = i.logId AND i.name LIKE ''%KW PRE%'' 
       WHERE l.itemId = ''',myItemId,'''
            AND completed BETWEEN ''', startDate, ''' AND ''', endDate, '''
      ORDER BY completed');   
    
    PREPARE stmt FROM @sql;
     EXECUTE stmt;
     DROP TEMPORARY TABLE tempTable3;
     DEALLOCATE PREPARE stmt;

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topRow.jpg
portlightAsked:
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Giuseppe PizzutoIT DirectorCommented:
Use Coalesce function.
I.e.
SELECT coalesce(coolingHours,completed) FROM ...

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portlightAuthor Commented:
I do not think this is going to work exactly they way I need it because the FIRST row is not always going to be null.

I only need to change the values if the first returned row is null.
SharathData EngineerCommented:
Can you provide some sample data from your tables and expected result
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MySQL Server

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