3路SQL加入返回错误数据

Question

我正在尝试使用以下sql来检索收到的总金额,以及从3个表中销售的总金额:

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

上面的查询返回的数据比实际值大几个数量级.但是,如果我把它分成两个查询:

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

它完美地运作.是什么赋予了？是否有任何原因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

如果任何一个receivings或sales具有多行(尤其是如果两者都具有多行),那么该值将分别获得由其他行的数量成倍增加.解决此问题的一种方法是分别计算总数然后加入:

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'