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.
一种办法是子查询，第二种使用 Having 更加简洁。
SELECT class FROM courses GROUP BY class HAVING COUNT(distinct student) >= 5