您需要三个独立的查询，连接在一起。

SELECT o."flight" AS "oneway_flight", 
       NULL AS "roundout_flight", 
       NULL."flight" AS "roundin_flight", 
       o.departure_time AS "oneway_departure_time", 
       NULL AS "roundout_departure_time",    
       NULL AS "roundin_departure_time", 
       o.arrival_time AS "oneway_arrival_time", 
       NULL AS "roundout_arrival_time", 
       NULL AS "roundin_arrival_time" 
  FROM "dbo"."data" "d" 
  LEFT JOIN "dbo"."ONEWAY_OUTBOUND" "o" ON d.product_id = o.product_id 
  WHERE d.product_id = 'TXXXXXXX'

 UNION ALL

     SELECT NULL AS "oneway_flight", 
       "ro"."flight" AS "roundout_flight", 
       NULL AS "roundin_flight", 
       NULL AS "oneway_departure_time", 
       ro.departure_time AS "roundout_departure_time",    
       NULL AS "roundin_departure_time", 
       NULL AS "oneway_arrival_time", 
       ro.arrival_time AS "roundout_arrival_time", 
       NULL AS "roundin_arrival_time" 
  FROM "dbo"."data" "d" 
  LEFT JOIN "dbo"."ROUNDTRIP_OUTBOUND" "ro" ON d.product_id = ro.product_id 
  WHERE d.product_id = 'TXXXXXXX'

 UNION ALL

 SELECT NULL AS "oneway_flight", 
       NULL AS "roundout_flight", 
       "ri"."flight" AS "roundin_flight", 
       NULL AS "oneway_departure_time", 
       NULL AS "roundout_departure_time",    
       ri.departure_time AS "roundin_departure_time", 
       NULL AS "oneway_arrival_time", 
       NULL AS "roundout_arrival_time", 
       ri.arrival_time AS "roundin_arrival_time" 
  FROM "dbo"."data" "d" 
  LEFT JOIN "dbo"."ROUNDTRIP_INBOUND" "ri" ON d.product_id = ri.product_id 
  WHERE d.product_id = 'TXXXXXXX'