如何使用具有不同值的COUNT（*）函数？

Question

例如，我的表格中包含geography和children等列。我需要计算每个childrens中有多少country。

我只能做出这样的事情：

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

嗯，我有例如：

Canada John
Canada John
Canada Peter

我需要来自加拿大的结果有2 children 但如果我要group by children，结果就是：

Canada 2
Canada 1

如果group by geography那么，

Canada 3

Answer 1

这里不需要

SUM。您唯一要做的就是按geography分组。

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

• See SQLFiddle Demo

Answer 2

如果您只需要计算组中的唯一值，则可以使用COUNT(DISTINCT expr)

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

Answer 3

如果地理位置是国家/地区字段，则必须将其添加到分组依据中。

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