mysql - 使用sql按组计算百分比

Question

我有一张表，其中包含学生的 ID、类(class)名称和类(class)级别。

+----+--------+-------+   
| Id | Course | Level |   
+----+--------+-------+   
| 1  | A      | 1     |    
| 2  | A      | 1     |   
| 1  | B      | 1     |   
| 3  | B      | 1     |   
| 4  | C      | 2     |   
+----+--------+-------+  

据此，我想知道每门类(class)按级别涵盖的百分比。

如下表所示:

+-------+--------+----------------+  
| Level | Course | Count_by_level |  
+-------+--------+----------------+  
| 1     | A      | 50%            |  
| 1     | A      | 50%            |     
| 1     | B      | 50%            |   
| 1     | B      | 50%            |   
| 2     | C      | 100%           |   
+-------+--------+----------------+    

我如何使用 SQL 执行此操作？

Answer 1

SQL DEMO

SELECT S.[Id] , S.[Course], S.[Level], T.ctotal,
       100.0 / T.ctotal
FROM students S
JOIN ( SELECT [Course], COUNT(*) as ctotal
       FROM students
       GROUP BY [Course]
      ) T
  ON S.[Course] = T.[Course]

输出

| Id | Course | Level | ctotal |     |
|----|--------|-------|--------|-----|
|  1 |      A |     1 |      2 |  50 |
|  2 |      A |     1 |      2 |  50 |
|  1 |      B |     1 |      2 |  50 |
|  3 |      B |     1 |      2 |  50 |
|  4 |      C |     2 |      1 | 100 |