MySQL query of Hotel database
SELECT r.rm_number
FROM   hotel h,
     reservation rr,
     room_booking b,
     room r
WHERE  rr.res_id = b.res_id
     AND b.rm_id = r.rm_id
     AND r.htl_id = h.htl_id
     AND h.htl_name = 'Holiday Inn Express'
     AND rr.res_checkin IS NOT NULL
     AND rr.res_checkout IS NULL ;