【问题标题】：SQL Grouped Table JoinSQL 分组表连接
【发布时间】：2019-03-09 05:44:22
【问题描述】：

我正在努力弄清楚如何连接两个分组表，然后执行减法运算

我有这些带有以下列的表格

表 1：购买

供应商 |数量 |价格

表 2：付款

供应商 |付费 |

结果表

供应商 |购买总额 | PaidTotal |（已购买 - 已支付）|

我非常不正确的 Sql 查询

 SELECT t.Vendor, SUM(Quantity * Price) as TotalPurch
    FROM purchases t
    LEFT JOIN (SELECT Vendor, SUM(paid) as TotalPaid FROM paysup GROUP BY Vendor) y
    ON t.Vendor = y.Vendor
    GROUP BY t.Vendor

【问题讨论】：

请使用标签来表明您正在使用哪个数据库，这很重要，因为“sql”不会告诉我们建议使用哪种方言作为解决方案。不同的数据库供应商在其 SQL 实现方面存在差异。

标签： sql join sum

【解决方案1】：

您可以使用公用表表达式 (CTE) 提高可读性：

WITH paid AS ( SELECT Vendor, SUM(Paid) AS PaidTotal
           FROM paysup
           GROUP BY Vendor
   ),
   purch AS ( SELECT Vendor, SUM(Quantity * Price) AS PurchasedTotal
          FROM purchases
          GROUP BY Vendor
   ) 
/* The main query: */
SELECT 
  v.Vendor, 
  purch.PurchasedTotal, 
  paid.PaidTotal, 
  (COALESCE(purch.PurchasedTotal, 0) - COALESCE(paid.PaidTotal, 0)) AS "(Purchased - Paid)"
 FROM Vendors v
  LEFT JOIN purch ON v.Vendor = purch.Vendor
  LEFT JOIN paid ON v.Vendor = paid.Vendor;

【讨论】：

【解决方案2】：

在这些情况下，通常会有一个您没有提及的供应商表

  SELECT v.Vendor
     , purch.PurchasedTotal
     , paid.PaidTotal
     , (COALESCE(purch.PurchasedTotal, 0) - COALESCE(paid.PaidTotal, 0)) AS "(Purchased - Paid)"
  FROM Vendors v
  LEFT JOIN ( SELECT Vendor, SUM(Quantity * Price) AS PurchasedTotal
              FROM purchases
              GROUP BY Vendor
             ) purch ON v.Vendor = purch.Vendor
  LEFT JOIN ( SELECT Vendor, SUM(Paid) AS PaidTotal
                FROM paysup
               GROUP BY Vendor
       ) paid ON v.Vendor = paid.Vendor
  ;

【讨论】：

【解决方案3】：

试试这个：

SELECT COALESCE(a.Vendor, b.Vendor) AS Vendor
     , a.PurchasedTotal
     , b.PaidTotal
     , (COALESCE(a.PurchasedTotal, 0) - COALESCE(b.PaidTotal, 0)) AS "(Purchased - Paid)"
  FROM ( SELECT Vendor, SUM(Quantity * Price) AS PurchasedTotal
           FROM purchases
          GROUP BY Vendor
       ) a
  FULL JOIN ( SELECT Vendor, SUM(Paid) AS PaidTotal
                FROM paysup
               GROUP BY Vendor
       ) b ON b.Vendor = a.Vendor

【讨论】：