Question

I was wondering if my solution of using a subquery in the following example is the most optimal way:

SELECT
    group_id,
    student_completed_lectures.student_id,
    count(student_completed_lectures.lecture_id) as completed_lectures_count
FROM group
LEFT JOIN group_students ON (group_students.group_id = group.id AND group_students.year = 2011)
LEFT JOIN student_completed_lectures ON (
    student_completed_lectures.student_id = group_students.student_id AND 
    student_completed_lectures.lecture_id IN ( 
        SELECT lecture_id 
        FROM  lecturesets
        WHERE lecturesets.id = group.lectureset_id ) )
GROUP BY student_completed_lectures.student_id

The idea is to count how many lectures a student has completed from what has been assigned to his group.

Answer 1

If you want to restrict the results you should do it like this

SELECT
group_id,
student_completed_lectures.student_id,
count(student_completed_lectures.lecture_id) as completed_lectures_count
FROM group
LEFT JOIN group_students ON (group_students.group_id = group.id AND group_students.year = 2011)
LEFT JOIN student_completed_lectures ON (
student_completed_lectures.student_id = group_students.student_id)
WHERE student_completed_lectures.lecture_id IN( 
    SELECT lecture_id 
    FROM  lecturesets
    WHERE lecturesets.id = group.lectureset_id )
GROUP BY student_completed_lectures.student_id