急，有关SQL连接问题！！！！！！

set rowcount 100
select a.sStoreNO,a.nGoodsID,(b.nTotalSaleAmount-a.nTotalSaleAmount) as '金额',(b.nTotalSaleQty-a.nTotalSaleQty) as '销量'
from tGoodsDaily as a ,tGoodsDaily as b
where a.sStoreNO='C503' and b.sStoreNO='C503' and a.dTradeDate = '20080218' and b.dTradeDate='20080219' and a.nGoodsID=b.nGoodsID
group by nGoodsID
order by (b.nTotalSaleAmount-a.TotalSaleQty) desc
set rowcount 0
报错：Server Message: Number 209, Severity 16
Server 'TONGCHENGSRVE', Line 2:
Ambiguous column name nGoodsID
Server Message: Number 207, Severity 16
Server 'TONGCHENGSRVE', Line 2:
Invalid column name 'TotalSaleQty'.

请高手指点！！！