如何使用具有不同值的COUNT（*）函數？

Question

例如，我有一個表格，它有geography和children這樣的列。我需要統計每個country有多少個childrens。

我只能做出這樣的事情：

SELECT geography, COUNT(*) 
FROM table1 
GROUP BY children 
ORDER BY geography 

嗯，我有，例如：

Canada John 
Canada John 
Canada Peter 

我需要的結果，例如加拿大有2個children。
但是如果我要group by children出現的結果是：

Canada 2 
Canada 1 

如果group by geography然後，

Canada 3 

Answer 1

如果需要只計算一個組中的唯一值，你可以使用COUNT(DISTINCT expr)

SELECT geography, COUNT(DISTINCT children) 
FROM table1 
GROUP BY geography 
ORDER BY geography 

Answer 2

SUM這裏並不是真的需要。你要做的唯一事情就是將它按geography分組。

SELECT geography, COUNT(DISTINCT children) totalCount 
FROM table1 
GROUP BY geography 
ORDER BY geography 

• See SQLFiddle Demo

Answer 3

如果地理學鄉間田野 - 你必須按組將它添加到。

SELECT geography, COUNT(children) 
FROM table1 
GROUP BY geography 
ORDER BY geography