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.

大概是考察group by的用法

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