尝试修改查询.以下行不应该是联接结构本身的一部分，而应该是联接结构的过滤器:

Try modifying the query. The following line should not be a part of the join structure itself, but rather a filter for the join structure:

OITW.WareHouseCode in ('WH1','WH2','WH4')

将其作为WHERE子句放入:

Put it in as a WHERE clause:

SELECT
   OITM.ItemCode,
   sum(StockHand - OrderQty) AS NetStockHand
FROM
   OITM inner join 
   WITM on OITM.ItemCode = WITM.ItemCode inner join 
   OITW on OITM.ItemCode = OITW.ItemCode 
WHERE 
   OITW.WareHouseCode in ('WH1','WH2','WH4')
GROUP BY 
   OITM.ItemCode 
HAVING
   sum(StockHand - OrderQty) <> WITM.LastUStock