从两个表中获取总和答案

【问题标题】：Getting sum from two tables从两个表中获取总和
【发布时间】：2016-05-18 16:54:57
【问题描述】：

我有两张桌子。

Voucher 
V_Id Cost
1     400
2     500

Expense 
E_Id  V_Id  Cost
1      1     100
2      1     100
3      1     100

我需要加入他们才能得到

V_Id  V_Cost  E_Cost
1      400     300
2      500      0

我尝试过 INNER JOIN，但我得到的值相同。请帮忙。

【问题讨论】：

标签： sql-server sql-server-2008 join sum

【解决方案1】：

我将 Expense 表的 SUM(Cost) 作为 E_Cost，然后用 Voucher Left join 得到预期的结果：

SELECT V.V_Id, V.Cost AS V_Cost, ISNULL(E.E_Cost, 0) AS E_Cost
FROM Voucher V
LEFT JOIN (  SELECT V_Id, SUM(Cost) AS E_Cost
             FROM Expense 
             GROUP BY V_Id ) AS E ON E.V_Id = V.V_Id

给定数据的工作示例：

DECLARE @Voucher TABLE (V_Id INT, Cost INT)
INSERT INTO @Voucher (V_Id, Cost)
VALUES (1, 400), (2, 500)

DECLARE @Expense TABLE (E_Id INT, V_Id INT, Cost INT)
INSERT INTO @Expense (E_Id, V_Id, Cost)
VALUES (1, 1, 100), (2, 1, 100), (3, 1, 100)

SELECT V.V_Id, V.Cost AS V_Cost, ISNULL(E.E_Cost, 0) AS E_Cost
FROM @Voucher V
LEFT JOIN (  SELECT V_Id, SUM(Cost) AS E_Cost
             FROM @Expense 
             GROUP BY V_Id ) AS E ON E.V_Id = V.V_Id

【讨论】：