Can't use sql lag sum : My Query is like this


    SELECT
    [day], [time],AvaliableTimes,
     case whEN AvaliableTimes >0 THEN sum(AvaliableTimes) OVER (
                          ORDER BY [day], [time],AvaliableTimes
                             )  else 0 END AS SumValue
FROM
    [AvailableTimes] 
where [day] = 1 AND BranchAreaId = 1
ORDER BY
     [day], [time],AvaliableTimes



I want to start sum from 0 if value is null or 0. 
Results:

    
 




  [Attached Image below][1]


  [1]: https://i.stack.imgur.com/v0RDJ.jpg