การ Where ของ SQL จะกำหนดว่าถ้า เป็น N หรือ W ให้แสดงค่า อย่างไรดีคะ

SELECT t.* FROM (
SELECT ROW_NUMBER() OVER(ORDER BY b.booking_no) AS rownum, 
  b.flight_booking_seq as flightBookingSeq,
  b.booking_no as bookingNo,
  isnull(b.first_name,'')+' '+isnull(b.last_name,'') as bookName,
  b.mobile_phone as mobilePhone,
  b.work_phone as workPhone,
   (SELECT COUNT(ck.check_in_seq)
				  FROM ka_check_in ck
				  WHERE ck.passenger_type <> 'I'
              AND  ck.flight_itinerary_seq = i.flight_itinerary_seq) as passengerNo,
  ei.item_desc as bookingStatus,
  i.flight_itinerary_seq as flightItinerarySeq,
  case b.round_trip 
  when 'Y' then 'true'
  else 'false'
  end as roundTrip,
  b.booking_note as bookingNote,
  b.email as email,
 dbo.get_check_in_status(i.flight_itinerary_seq) as checkInStatus,
  b.booking_status,
  isnull(dbo.get_count_bag_booking(i.flight_itinerary_seq),'') as bagNoBook,
  dbo.get_booking_by(b.flight_booking_seq) as bookingBy
from ka_flight_sche s , ka_flight_itinerary i , ka_flight_booking b , sc_entry_item ei
where s.flight_sche_seq = i.flight_sche_seq
  and i.flight_booking_seq = b.flight_booking_seq
  and s.flight_sche_seq = '5179'
  and ei.param_code = 'KA0001'
  and ei.entry_code = '1'
  and ei.item_code = b.booking_status
  and b.booking_status = 'N'
  
) as t