596.超过5名学生的课

解析

select class from courses
group by class 
having
    count( distinct student) >= 5;

之前直接考虑的是，虽然结果相同但是跟题目不符，注意“学生在每个课中不应被重复计算”

select class from courses
group by class 
having
    count( class) >= 5;