SQL

sql add column int nullable

ALTER TABLE YourTable
  ADD Foo INT NULL /*Adds a new int column existing rows will be 
                     given a NULL value for the new column*/

add values of column with null sql

SELECT sum(COALESCE(TotalHoursM,0))
          + COALESCE(TotalHoursT,0)
          + COALESCE(TotalHoursW,0)
          + COALESCE(TotalHoursTH,0)
          + COALESCE(TotalHoursF,0) AS TOTAL FROM LeaveRequest