Question

I have an application where users can take tests (which are composed of questions and answers).

I’m trying to construct a query that returns a count of answers grouped by question, for a specific teacher and test.
The problem is I would like the query to return 0 count for questions with no answers.

answers

• id
• question_id
• test_id
• student_id

questions

• id

teacher_students

• student_id
• teacher_id

tests

• id

Query

   SELECT COUNT(answers.id) AS rcount,
          questions.id 
     FROM "questions"
LEFT JOIN answers ON answers.question_id = questions.id
LEFT JOIN teacher_students ON teacher_students.student_id = answers.student_id
    WHERE (questions.test_id = 1)  
      AND (teacher_students.teacher_id = 1)
 GROUP BY questions.id 
 ORDER BY questions.id

Output

 rcount | question_id 
--------+----
      4 | 1
      2 | 3

Desired Output

 rcount | question_id 
--------+----
      4 | 1
      0 | 2
      2 | 3
      0 | 4

If I remove teacher_students.teacher_id = 1, the number of questions returned is correct, but the count is not.

Answer 1

Move the teacher_students.teacher_id = 1 check from the WHERE clause to the joining ON clause.

When a condition that refers to a table in the right side of a LEFT JOIN is put in the WHERE clause, the LEFT JOIN is cancelled and it acts as an INNER JOIN.

SELECT count(teacher_students.student_id) AS rcount      <--- changed
     , questions.id 
FROM "questions"
  LEFT JOIN answers 
    ON answers.question_id = questions.id
  LEFT JOIN teacher_students 
    ON teacher_students.student_id = answers.student_id
    AND teacher_students.teacher_id = 1
WHERE questions.test_id = 1

GROUP BY questions.id 
ORDER BY questions.id