Prova något sånt här
SELECT
tbl_room.room_no,
tbl_room.type_id,
tbl_room.id,
tr.id as reserv_status
FROM tbl_room
LEFT JOIN tbl_Reservation tr ON tbl_room.id = tr.room_id AND
(
(tr.checkin_data <= "'$checkin'" AND tr.checkout_data >= "'$checkin'")
OR
(tr.checkin_data <= "'$checkout'" AND tr.checkout_data >= "'$checkout'")
)
WHERE
tbl_room.type_id =1 AND tr.id IS NULL