计算研究区域的百分比

Question

我需要生成SQL数据库的帮助。

我有两个表：

• 学生
• student_Subjects

学生表：

 
+------------+---------+-----------------+ 
| id  | name |area_of_study_id | 
+------------+---------+-----------------+ 
| 1   | AAA  | 1    | 
| 2   | BBB  | 2    | 
| 3   | CCC  | 1    | 
| 4   | DDD  | 3    | 
| 5   | EEE  | 4    | 
| 6   | FFF  | 1    | 
| 7   | GGG  | 2    | 
| 8   | III  | 1    | 
+------------+---------+-----------------+ 

student_subjects表：

 
+------------+-------------------+------------------+ 
| id  | student_id  | subject_id  | 
| 1   | 1     |  1   | 
| 2   | 2     |  1   | 
| 3   | 1     |  3   | 
| 4   | 1     |  2   | 
| 5   | 3     |  1   | 
| 6   | 6     |  1   | 
| 7   | 7     |  3   | 
| 8   | 4     |  6   | 
+------------+-------------------+------------------+ 

我需要找出基于students area of study一个subject's area of study。从学生表上可以看到，学生可以自由选择area of study。有时，他们可以选择与其学习领域无关的科目。因此，在这种情况下，我们需要根据已采纳主题的student percentages来计算area of study的subject。

的部分输出：

 
+------------+-------------------+-----------------------------------+ 
|Total_students | subject_id |area_of_study_id | percentage  | 
+------------+-------------------+-----------------+-----------------+ 
| 3   | 1     | 1    | 75    | 
| 1   | 1     | 2    | 25    | 
| 1   | 3     | 1    | 50    | 
| 1   | 3     | 2    | 50    | 
| 1   | 2     | 1    | 100    | 
| 1   | 6     | 3    | 100    | 
+------------+-------------------+-----------------+-----------------+ 

从上面的部分输出中，我们可以看到，3名学生选择的受试者（即subject_id = 1）具有研究1的面积和一个学生是来自不同区研究（即2）。所以，我们可以说subject_id =1是从area of study = 1。

如果百分比50％为一个主题。我们可以选择两个之间的任何学习领域，不管是哪一个。

预期输出：

 
+------------+-------------------+-----------------------------------+ 
|Total_students | subject_id |area_of_study_id | percentage  | 
+------------+-------------------+-----------------+-----------------+ 
| 3   | 1     | 1    | 75    | 
| 1   | 3     | 1    | 50    | 
| 1   | 2     | 1    | 100    | 
| 1   | 6     | 3    | 100    | 
+------------+-------------------+-----------------+-----------------+ 

在先进的感谢。

Answer 1

尝试这种方式

SELECT subject_id, area_of_study_id, total_students, percentage 
    FROM 
(
    SELECT p.subject_id, p.area_of_study_id, p.total_students, 
     p.total_students/t.total_students * 100 percentage 
    FROM 
    (
    SELECT COUNT(*) total_students, ss.subject_id, s.area_of_study_id 
     FROM students s JOIN student_subjects ss 
     ON s.id = ss.student_id 
    GROUP BY ss.subject_id, s.area_of_study_id 
) p JOIN 
    (
    SELECT ss.subject_id, COUNT(*) total_students 
     FROM students s JOIN student_subjects ss 
     ON s.id = ss.student_id 
    GROUP BY ss.subject_id 
) t ON p.subject_id = t.subject_id 
    ORDER BY percentage DESC 
) q 
    GROUP BY subject_id; 

输出：

 
| SUBJECT_ID | AREA_OF_STUDY_ID | TOTAL_STUDENTS | PERCENTAGE | 
|------------|------------------|----------------|------------| 
|   1 |    1 |    3 |   75 | 
|   2 |    1 |    1 |  100 | 
|   3 |    1 |    1 |   50 | 
|   6 |    3 |    1 |  100 | 

这里是SQLFiddle演示