Postgresql联接三表

我有一个库存数据库，我需要通过我的库存计算每次买卖操作后的产品数量。我有三张表，方程如下：

(QtyInital +     (BuyQuantity(IN)  - SELLQUANTITY(OUT) 

这是我的三个表的模式

product(pid,name,qteInital,qteStock);
productInBuyFacture(baid,pid,qte,price,subtotal);
productInSellFacture(bsid,pid,qte,price,subtotal);

我想通过触发器计算当前的库存量。我试着通过这样的子查询

select ((select qteInital from product where id = 3) + 
(select qte from productInBuyFacture where pid = 3 ) - 
(select qte from productInSellFacture where pid = 3) as currentQuantity ; 

---

我的猜测是，你需要求和并修正括号，使它们平衡：

select ((select coalesce(sum(qteInital), 0) from product where id = 3) + 
        (select coalesce(sum(qte), 0) from productInBuyFacture where pid = 3 ) - 
        (select coalesce(sum(qte), 0) from productInSellFacture where pid = 3)
       ) as currentQuantity ; 

---

coalesce（）

用于防止出现不匹配的问题。算术表达式中的

NULL

通常会导致整个表达式返回

NULL

您也可以尝试使用显式联接：

SELECT pro.pid, pro.QtyInital, COALESCE(SUM(buy.qte), 0) AS buyqty, COALESCE(SUM(sell.qte), 0) AS sellqty
FROM product AS pro
LEFT JOIN productInBuyFacture AS buy
ON pro.pid = buy.pid
LEFT JOIN productInSellFacture AS sell
ON pro.pid = sell.pid
GROUP BY pro.pid, pro.QtyInital

---

当你试着那样做的时候发生了什么？