sql12（Leetcode1280学生们参加各科测试的次数）

代码：

好复杂的简单题

cross join

# Write your MySQL query statement below
select
    s.student_id,
    s.student_name,
    sub.subject_name,
    ifnull(grouped.attended_exams,0) as attended_exams
from 
    Students s
cross join
    Subjects sub
left join(
    select 
        student_id, 
        subject_name,
        count(*) as attended_exams
    from 
        Examinations
    group by 
        student_id,subject_name)grouped
on s.student_id = grouped.student_id and sub.subject_name = grouped.subject_name
order by s.student_id,sub.subject_name