解决

left join on 通过两个或多个字段才可以关联到一条数据

ON concat( o.to_id, o.from_id )= concat( r.to_id, r.from_id )

查询如下

SELECTo.id,r.to_id, r.from_id,r.distance
FROMmy_order AS oLEFT JOIN my_route AS r ON concat( o.to_id, o.from_id )= concat( r.to_id, r.from_id ) 
WHEREo.id IN (15939,15938,15937,);