我如何在mysql工作台中找到此查询

Question

For each student, find the number of courses they take and sort the rows in descending order. (eg student id, the number of courses taken by that student)

STUDENT TABLE

| ID      | name      | dept_name      | tot_cred |
| S0901   | Alice     | Comp.Sci.      | 83       |
| S0902   | Martha    | Comp.Sci.      | 75       |
| S0903   | Micheal   | Comp.Sci.      | 45       |
| S0904   | Rose      | Comp.Sci.      | 77       |
| S0905   | Alfie     | Comp.Sci.      | 91       |
| S1901   | Brad      | Biology        | 23       |

TAKES TABLE

| ID      | course_id      | sec_id      | semester      | year      | grade      
| S0901   | CS-101         | 1           | Fall          | 2009      | A          
| S0901   | CS-315         | 1           | Spring        | 2010      | B+         
| S0901   | HIS-351        | 1           | Spring        | 2010      | A-         
| S0901   | MTH-101        | 1           | Fall          | 2009      | A          
| S0901   | MTH-102        | 1           | Spring        | 2009      | B+   
| S0902   | CS-101         | 1           | Fall          | 2009      | A          
| S0902   | CS-315         | 1           | Spring        | 2010      | B+         
| S0902   | CS-319         | 1           | Spring        | 2010      | B          
| S0902   | HIS-351        | 1           | Spring        | 2010      | A-         
| S0902   | MTH-101        | 1           | Fall          | 2009      | A          
| S0902   | MTH-102        | 1           | Spring        | 2009      | B+         
| S1901   | CS-101         | 1           | Fall          | 2009      | B+         
| S1901   | CS-190         | 1           | Spring        | 2009      | C          
| S1901   | CS-315         | 1           | Spring        | 2010      | A-         
| S1901   | HIS-351        | 1           | Spring        | 2010      | A-    

Answer 1

Technically, @Marcinek's answer may not be sufficient because it omits students who take zero classes. I would use this instead:

SELECT STUDENT.ID, COUNT(TAKES.ID)
FROM STUDENT LEFT JOIN TAKES ON STUDENT.ID = TAKES.ID
GROUP BY STUDENT.ID
ORDER BY COUNT(TAKES.ID) DESC;

By using LEFT JOIN , you can capture a student whose ID does not appear in the TAKES table.

Answer 2

只需将结果分组即可。

SELECT COUNT(*), s.id FROM student s, takes t where t.id = s.id group by s.id order by count(*) desc