[sql] SQL: sum 3 columns when one column has a null value?

looks like you want to SUM all the columns (I'm not sure where "sum 3 columns" comes from), not just TotalHoursM, so try this:

SELECT 
    SUM(    ISNULL(TotalHoursM  ,0)
          + ISNULL(TotalHoursT  ,0)
          + ISNULL(TotalHoursW  ,0)
          + ISNULL(TotalHoursTH ,0)
          + ISNULL(TotalHoursF  ,0) 
       ) AS TOTAL
    FROM LeaveRequest