SUM IF查詢多表

Question

我有一個表，每個月有一列我需要添加。我需要做一個查詢來將它全部顯示在一張表中。我做的第一件事就是將所有表聯合起來以獲得獨特的國家。如何配置此ms訪問查詢，我開始產生我需要的輸出？

實施例表：

表1：2014年1月

| Country | Headcount | 
| USA | 100 | 
| China |  50 | 
| Russia |  75 | 
| USA |  10 | 

表2：2014年2月

| Country | Headcount | 
| USA | 100 | 
| China |  50 | 
| Mexico |  100 | 
| China |  75 | 

表3：全部國家

| Country | 
| USA | 
| China | 
| Mexico | 
| Russia | 

所需的輸出：

| Country | January | February | 
| USA | 110 | 100  | 
| China | 50  | 125  | 
| Russia |  75 | 0   | 
| Mexico |  0  | 100  | 

這是我的查詢，到目前爲止，但它似乎並沒有工作..

SELECT a.[Country] As Country, 
SUM(IIF(a.[Country] = b.[Country], b.[Headcount],0)) As January 
SUM(IIF(a.[Country] = c.[Country], c.[Headcount],0)) As February 
FROM [All Countries] As a, 
(SELECT [Headcount], [Country] FROM [January 2014]) As b, 
(SELECT [Headcount], [Country] FROM [February 2014]) As c 
GROUP BY a.[Country] 

Answer 1

這是一個辦法，應該工作 - （編輯）

SELECT a.[Country] As Country, 
SUM(IIF(a.[Month] = "Jan", b.[Headcount],0)) As January 
SUM(IIF(a.[Month] = "Feb", c.[Headcount],0)) As February 

FROM  (
SELECT [Headcount], [Country], "Jan" AS [Month] FROM [January 2014] 
UNION ALL 
SELECT [Headcount], [Country], 'Feb' AS [Month] FROM [February 2014] 
UNION ALL 
... 
) AS a 
GROUP BY a.[Country] 

Answer 2

我會嘗試這樣的事：

SELECT a.country, 
(SELECT SUM(b.headcount) FROM table1 b WHERE b.country = a.country) as january, 
(SELECT SUM(c.headcount) FROM table2 c WHERE c.country = a.country)as february 
FROM countries a 

GROUP BY a.`country`