问题陈述链接
正确代码（董玉章）：

select con.contest_id,
        con.hacker_id, 
        con.name, 
        sum(total_submissions), 
        sum(total_accepted_submissions), 
        sum(total_views), sum(total_unique_views)
from contests con 
join colleges col on con.contest_id = col.contest_id 
join challenges cha on  col.college_id = cha.college_id 
left join
(select challenge_id, sum(total_views) as total_views, sum(total_unique_views) as total_unique_views
from view_stats group by challenge_id) vs on cha.challenge_id = vs.challenge_id 
left join
(select challenge_id, sum(total_submissions) as total_submissions, sum(total_accepted_submissions) as total_accepted_submissions from submission_stats group by challenge_id) ss on cha.challenge_id = ss.challenge_id
    group by con.contest_id, con.hacker_id, con.name
        having sum(total_submissions)!=0 or 
                sum(total_accepted_submissions)!=0 or
                sum(total_views)!=0 or
                sum(total_unique_views)!=0
            order by contest_id;

我修改了代码，但没有子查询，这是不正确的，并且给出了更大的和值。我不明白写子查询有什么不同？一个简单的测试用例示例将非常有用。谢谢！

select con.contest_id,
        con.hacker_id, 
        con.name, 
        sum(total_submissions), 
        sum(total_accepted_submissions), 
        sum(total_views), sum(total_unique_views)
    from contests con 
    join colleges col on con.contest_id = col.contest_id 
    join challenges cha on  col.college_id = cha.college_id 

        left join view_stats vs 
        on cha.challenge_id = vs.challenge_id 
        left join submission_stats ss
        on cha.challenge_id = ss.challenge_id

    group by con.contest_id, con.hacker_id, con.name
    having sum(total_submissions)!=0 or 
    sum(total_accepted_submissions)!=0 or
    sum(total_views)!=0 or
    sum(total_unique_views)!=0
    order by contest_id;