[LeetCode] Classes More Than 5 Students

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.

# Write your MySQL query statement below
SELECT class FROM courses GROUP BY class HAVING COUNT(DISTINCT student) >= 5;
# 2772 ms

# Write your MySQL query statement below
SELECT class FROM (SELECT class,COUNT(DISTINCT student) AS num FROM courses GROUP BY class) AS temp_table WHERE num >= 5;
# 2666 ms