【leetcode-数据库】Classes More Than 5 Students 课程学生数大于5

There is a table courses with columns: student and class

Please list out all classes which have more than or equal to 5 students.

For example, the table:

+---------+------------+
| student | class      |
+---------+------------+
| A       | Math       |
| B       | English    |
| C       | Math       |
| D       | Biology    |
| E       | Math       |
| F       | Computer   |
| G       | Math       |
| H       | Math       |
| I       | Math       |
+---------+------------+

Should output:

+---------+
| class   |
+---------+
| Math    |
+---------+

Note:
The students should not be counted duplicate in each course.

解法一： select courses_cnt.class from (select count(DISTINCT student)as cnt,class from courses where 1=1 group by class) as courses_cnt where courses_cnt.cnt >= 5

 

解法二：分组

select c.class
from courses c
group by c.class
having count(DISTINCT c.student)>4