如何在 SQL 中使用多个 COUNT 语句共享同一个 GROUP BY？

Question

我有以下语句输出正确的数据

 SELECT COUNT (Section_ID) AS Sections
 FROM Section
 GROUP BY Location;

而且这条语句也输出了正确的数据

 SELECT COUNT (e.Section_ID) AS Students
 FROM Section s, Enrollment e
 WHERE s.Section_ID = e.Section_ID
 GROUP BY Location;

然而，当我把这些陈述像这样放在一起时

 SELECT Location, COUNT(s.Section_ID) AS Sections, COUNT (e.Section_ID) AS Students
 FROM Section s, Enrollment e
 WHERE s.Section_ID = e.Section_ID
 GROUP BY Location;

部门和学生现在共享相同的数据。如何将这些语句组合在一起？

Answer 1

首先，不要在FROM 子句中使用逗号。 始终使用正确、明确的JOIN 语法。

其次，记住COUNT() 做了什么。它计算非NULL 值的数量。

做你想做的事情的一种方法是使用COUNT(DISTINCT)：

SELECT Location, COUNT(DISTINCT s.Section_ID) AS Sections,
       COUNT(DIStINCT e.Student_Id) AS Students
FROM Section s JOIN
     Enrollment e
     ON s.Section_ID = e.Section_ID
GROUP BY Location;

我必须猜测Enrollment 的主键是什么。我猜Student_ID。