Pour retourner ou produire NULL valeurs que vous devrez utiliser LEFT JOIN s.
Ainsi, votre requête devrait ressembler à :
SELECT
a.ReservationStayID AS 'Reservation Id'
,a.PMSConfirmationNumber AS 'PMS No'
,a.CreatedOn AS 'Date Created'
,a.ArrivalDate AS 'Date of Arrival'
,a.DepartureDate AS 'Date of Departure'
,a.TAProfileID AS 'TA Id'
,a.StatusCode AS 'Status'
,b.PropertyCode AS 'Hotel'
,c.NAME AS 'Travel Agency'
,c.CountryCode AS 'Market Code'
,d.CountryName AS 'Mkt'
FROM ReservationStay a
INNER JOIN GuestStaySummary b ON a.ReservationStayID = b.ReservationStayID
LEFT JOIN TravelAgency c ON a.TAProfileID = c.TravelAgencyID
LEFT JOIN Market d ON c.CountryCode = d.CountryCode
