Question

我想知道在以下示例中使用子查询的解决方案是否是最佳方式：

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

这个想法是计算一个学生从已经分配给他的小组的课程中完成了多少次课程。

Answer 1

如果你想限制结果，你应该这样做

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

何时（不）在JOIN语句中使用子查询？

1 个答案: