Question

Is it possible to perform a count based on a combination of two columns?

I have 3 tables:

student:
Student id | Student name | Gender | Dob |

class register: Student id | Class id

Both columns make up the composite primary key.

Classes: Class id | Class name | Class day

I would like to perform a count of the number of students of each gender in each class.

I came up with this query so far:

SELECT class_name AS 'class Name',
  Child_gender AS 'Gender',
  COUNT(student_id) AS 'Count'
FROM student,classRegister,classes
WHERE student.student_id=classRegister.student_id AND
  classRegister.class_id=classes.class_id
GROUP BY class_name,student_gender
ORDER BY class_name;

But it seems to be giving me duplicate results for some of the students.

So, can I modify this query to count the number of instances of a combination of the student_id and class_id?

I’m still very new to MySQL, so I apologise if this is basic for everyone

Answer 1

Try this:

SELECT c.class_name AS 'class Name', s.Child_gender AS 'Gender',
       COUNT(s.student_id) AS 'Count'
FROM student s
INNER JOIN classRegister cr ON s.student_id = cr.student_id 
INNER JOIN classes c ON cr.class_id = c.class_id
GROUP BY s.Child_gender, s.class_id, c.class_name
ORDER BY c.class_name