Say I have the following tables,
lesson
id description
-----------------
1 science
2 english
3 maths
lesson_student_rel
lesson_id student_id
----------------------
1 1
1 2
1 3
2 1
3 1
3 3
lesson_tutor_rel
lesson_id tutor_id
--------------------
1 1
2 1
2 2
lesson_assistant_rel
lesson_id assistant_id
------------------------
1 1
1 2
3 2
I would like to count the total number of students, tutors and assistants for each lesson so I get a result like,
lesson_id student_count tutor_count assistant_count
---------------------------------------------------------
1 3 1 2
2 1 2 0
3 2 0 1
I think I've been looking at this too long now and have got completely stuck, I've found a few similar questions but nothing that seems to answer this. Is it possible to do with one query? Any pointers would be good - an answer would be amazing.
Thanks!
I think this does it for you:
SELECT l.id,COUNT(DISTINCT lsr.student_id) student_count,COUNT(DISTINCT ltr.tutor_id) tutor_count
,COUNT(DISTINCT lar.assistant_id) assistant_count
FROM lesson l
LEFT JOIN lesson_student_rel lsr ON lsr.lesson_id=l.id
LEFT JOIN lesson_tutor_rel ltr ON ltr.lesson_id=l.id
LEFT JOIN lesson_assistant_rel lar ON lar.lesson_id=l.id
GROUP BY l.id
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.