请教SQL问题---------为什么不存在的记录也查询出来了?

SELECT 
	a.InNo,
	a.WeighNo,
	a.PartID,
	a.PartName,
	b.FullName
FROM InStoreSub a
INNER JOIN Weigh b ON a.WeighNo=b.WeighNo

--写法1：
select a.InNo,a.WeighNo,a.PartID,a.PartName
	,FullName=(select top 1 FullName from Weigh where WeighNo=a.WeighNo)
from InStoreSub a

--写法2：
select a.InNo,a.WeighNo,a.PartID,a.PartName,b.FullName from InStoreSub a,
	(select distinct WeighNo,FullName from Weigh) b
where a.WeighNo=b.WeighNo

--写法3：
select distinct a.InNo,a.WeighNo,a.PartID,a.PartName,b.FullName from InStoreSub a,Weigh b where a.WeighNo=b.WeighNo