如何使用具有不同值的COUNT（*）函數？

Question

例如，我的表具有諸如geography和children類的列。 我需要計算每個country有多少個childrens 。

我只能做這樣的事情：

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

好吧，我有例如：

Canada John
Canada John
Canada Peter

我需要從加拿大來的結果，例如有2個children 。
但是，如果我要按group by children結果是：

Canada 2
Canada 1

如果group by geography ，

Canada 3

Answer 1

SUM在這里並不需要。 您唯一要做的就是按geography將其分組。

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

• 請參見SQLFiddle演示

Answer 2

如果只需要計算組中的唯一值，則可以使用COUNT(DISTINCT expr)

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

Answer 3

如果地理位置是國家/地區字段-您必須將其添加到分組依據。

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