有一個courses 表 ，有: student (學生) 和 class (課程)。

請列出所有超過或等於5名學生的課。

例如,表:

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

應該輸出:

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

Note:
學生在每個課中不應被重複計算。

 第一種：

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

執行程式碼：

第二種：

select class from (select class,count(distinct student) as nums from courses group by class)b
where nums
 
>=5

 輸出：