CODEWITHSUNDEEP
sqlpostgresql
Josef_the_third
Josef_the_third

Reputation: 121

How can I count one-to-many relationship, but include 0's?

I essentially have 2 models:

Students Classes

Each student can sign up for as many classes as they want. I wrote a simple query to count how many classes each student has signed up for, but cannot figure out how to include students who have chosen 0 classes. I am ultimately trying to see how many students have signed up for how many classes. 

SELECT classes_chosen, count(*) AS student_count 
FROM (SELECT students.email, count(*) AS classes_chosen
  FROM students
  JOIN students_classes ON students_classes.student_id = students.id
  GROUP BY student_id) AS sub_q
GROUP BY classes_chosen
ORDER BY classes_chosen ASC;

I would like to end with something like:

    classes_chosen, student_count
    0, 47
    1, 35
    2, 45
    3, 85
    .
    .
.

Upvotes: 0

Views: 83

Answers (2)

forpas
forpas

Reputation: 164139

First aggregate inside students_classes only to get how many classes has each student chosen and then LEFT join students to the results and aggregate again:

SELECT COALESCE(classes_chosen, 0) classes_chosen, COUNT(*) AS student_count 
FROM students s 
LEFT JOIN (
  SELECT student_id, COUNT(*) AS classes_chosen
  FROM students_classes 
  GROUP BY student_id
) AS c ON c.student_id = s.id
GROUP BY classes_chosen
ORDER BY classes_chosen ASC;

See a simplified demo.

Upvotes: 1

Gordon Linoff
Gordon Linoff

Reputation: 1270443

You can basically use LEFT JOIN in the subquery:

SELECT classes_chosen, count(*) AS student_count 
FROM (SELECT students.email, count(sc.student_id) AS classes_chosen
      FROM students s LEFT JOIN
           students_classes s
           ON sc.student_id = s.id
      GROUP BY student_id
     ) AS sub_q
GROUP BY classes_chosen
ORDER BY classes_chosen ASC;

Note: If all students have a matching class, then you will not get 0 in the result set. I interpret your question as wanting to count all students rather than particularly returning 0.

Upvotes: 1

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