select
  c.Part_Num,
  a.InCount as inCount,
  b.OutCount as outCount
from c
left join ((select part_Num,sum(inCount) as inCount from a group by part_Num)) as a on c.Part_Num=a.Part_Num
left join (select part_Num,sum(outCount) as outCount from b group by part_Num) as b on b.Part_Num=a.Part_Num
group by c.Part_Num

------解决方案--------------------

SQL code
select Part_Num,sum(InCount),sum(OutCount)
from (
select Part_Num,InCount,0 as OutCount from a
union all
select Part_Num,0 as InCount,OutCount from b
union all
select Part_Num,0,0 from c
)
group by Part_Num