何時（不）在JOIN語句中使用子查詢？

Question

我在想，如果我在下面的例子中使用子查詢的解決方案是最優化的方式：

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 

的想法是算多少講課學生已經從過去已經分配給他的小組完成。

Answer 1

如果你想限制的結果，你應該做這樣的

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