3路SQL加入返回坏数据

Question

我尝试使用下面的SQL检索总量接收，总量从3个表销售：

select ITEMS.ITEM_NO, SUM(RECEIVINGS.QTY_RECVD), SUM(SALES.QTY_SOLD) 
from ITEMS 
left join RECEIVINGS 
on ITEMS.ITEM_NO = RECEVINGS.ITEM_NO 
left join SALES 
on ITEMS.ITEM_NO = SALES.ITEM_NO 
WHERE ITEMS.ITEM_NO = '12345' 
GROUP BY ITEMS.ITEM_NO 

以上的回报数据的查询是比实际值高几个数量级。但是，如果我将它分解为两个查询：

select ITEMS.ITEM_NO, SUM(RECEIVINGS.QTY_RECVD) 
from ITEMS 
left join RECEIVINGS 
on ITEMS.ITEM_NO = RECEVINGS.ITEM_NO 
WHERE ITEMS.ITEM_NO = '12345' 
GROUP BY ITEMS.ITEM_NO 


select ITEMS.ITEM_NO, SUM(SALES.QTY_SOLD) 
from ITEMS 
left join SALES 
on ITEMS.ITEM_NO = SALES.ITEM_NO 
WHERE ITEMS.ITEM_NO = '12345' 
GROUP BY ITEMS.ITEM_NO 

它完美地工作。是什么赋予了？是否有任何理由加入所有三个表的第一个查询将无法工作，但两个单独的查询将？

Answer 1

如果receivings或sales有多行（尤其是如果两行都有多行），那么这些值将分别乘以另一行中的行数。解决这个问题的一种方法是分别计算总量，然后加入：

select ITEMS.ITEM_NO, COALESCE(R.SUM_RECEIVINGS,0), COALESCE(S.SUM_SALES,0) 
from ITEMS 
left join (SELECT RECEIVINGS.ITEM_NO, SUM(RECEIVINGS.QTY_RECVD) as SUM_RECEIVINGS 
    FROM RECEIVINGS 
    GROUP BY RECEIVINGS.ITEM_NO) as R ON ITEMS.ITEM_NO = R.ITEM_NO 
left join (SELECT SALES.ITEM_NO, SUM(SALES.QTY_RECVD) as SUM_SALES 
    FROM SALES 
    GROUP BY SALES.ITEM_NO) as S ON ITEMS.ITEM_NO = S.ITEM_NO 
WHERE ITEMS.ITEM_NO = '12345'