mysql - 如何使用具有不同值的 COUNT(*) 函数？

Question

例如，我有一个表，其中包含 和 等geography列children。childrens我需要计算每个有多少country。

我只能做这样的事情：

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

好吧，我有例如：

Canada John
Canada John
Canada Peter

我需要结果，例如来自 Canada 有 2 children。
但如果我会这样做group by children，结果是：

Canada 2
Canada 1

如果group by geography那时，

Canada 3

Answer 1

SUM这里不是真的需要。您唯一要做的就是将其分组geography。

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

• 请参阅 SQLFiddle 演示

Answer 2

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

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

Answer 3

如果地理是国家/地区字段 - 您必须将其添加到分组依据。

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