在一个查询中计算多列

Question

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!

Answer 1

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