如何查找组中另一组的计数

Question

我无法在标题中清楚地解释问题，但这就是问题所在。

我正在使用 SQL Server。

我有一张包含大学和学科的表格。

University     Subject

Melbourne      Math
Melbourne      Physics
Melbourne      English

Ottawa         Physics
Ottawa         English
Ottawa         Arts
Ottawa         Computer

Sydney         Math
Sydney         Medicine
Sydney         English
Sydney         Computer
.
.
.

我想在一个 select 语句中做一些计算

这是我的 SQL

SELECT Subject, Count(*) SubjectsCount
FROM Table
GROUP BY Subject

给予

Subject SubjectsCount
Math       39
Physics    23
English    47
Medicine   13

我想在同一个查询中找到不同大学的数量

Subject SubjectsCount UniversitiesNo 
Math       39           52
Physics    23           52
English    47           52
Medicine   13           52

我试过了

SELECT Subject, Count(*) SubjectsCount, Count(*) OVER() AS UniversitiesNo
FROM Table
GROUP BY Subject

但这给了所有记录计数

我怎样才能为 Universities 指定唯一不同的？

Answer 1

您可以交叉连接一个返回所有不同大学数量的查询：

SELECT t.Subject, 
       COUNT(*) SubjectsCount,
       u.UniversitiesNo 
FROM tablename t 
CROSS JOIN (SELECT COUNT(DISTINCT University) UniversitiesNo FROM tablename) u
GROUP BY t.Subject, u.UniversitiesNo

甚至更简单：

SELECT Subject, 
       COUNT(*) SubjectsCount,
       (SELECT COUNT(DISTINCT University) FROM tablename) UniversitiesNo 
FROM tablename
GROUP BY Subject

请参阅demo。

Answer 2

使用COUNT DISTINCT:

SELECT Subject
   , Count(*) SubjectsCount
   , Count(DISTINCT University) AS UniversitiesNo
FROM Table
GROUP BY Subject

此查询给出每个Subject 的不同大学的数量。

要获得不同数量的大学，只需CROSS JOIN：

SELECT t1.Subject,
       t1.SubjectsCount,
       t2.cnt
FROM (       
   SELECT Subject, Count(*) SubjectsCount         
   FROM Table1  
  GROUP BY Subject
) AS  t1
CROSS JOIN (SELECT COUNT(DISTINCT University) FROM Table1) AS t2(cnt)

Demo here