Mysql的加入并计算每个类别

Question

您好我有这样
考试一表

id | exam_name 
1 | computer science 
2 | Environment science 

exam_students

id | exam_id | student_name 
1 | 1  | Josh 
2 | 1  | Michael 
3 | 1  | John 

我只需要加入和计算的总学生每个考试和输出这样的东西

exam_name   | total_students | 
computer science | 3    | 
Environment science| 0    | 

感谢您的帮助和建议

Answer 1

试试这个

SELECT 
    a.exam_name, count(b.id) AS total_students 
FROM 
    exams a 
LEFT JOIN exam_students b ON a.id = b.exam_id 
GROUP BY 
    a.id 

希望这有助于