จะต้องทำอย่างไรให้ค่าที่ถูกคำนวณมาก่อนหน้า นำมาคำนวณต่อได้ใน SQL ค่ะ [แนะนำด้วยคะ!!]

SELECT s.flight_sche_seq as pkCode,
       f.flight_no+'  '+f.flight_desc+' ' as flightNo, 
       f.flight_dep as departure,
       f.flight_arr as arrival,
       (SELECT r.route_name FROM ka_route r WHERE r.route_code = 'CNX') as depFrom,
       (SELECT r.route_name FROM ka_route r WHERE r.route_code = 'HGN') as arrTo,
       f.price as price,
       e.route_airport_tax as APT,
       f.price - e.route_airport_tax as TotalVat,
       TotalVat * 7 / 107 as Cost
  FROM ka_route e, ka_flight_sche s LEFT JOIN ka_flight f 
    ON s.flight_no = f.flight_no
 WHERE convert(varchar, s.flight_date, 103) = '25/08/2012'
   AND f.flight_from = 'CNX'
   AND f.flight_to = 'HGN'
   AND s.aval_seat >= '1'
   and e.route_code = 'CNX'