题目：

有一个courses 表 ，有: student (学生) 和 class (课程)。

请列出所有超过或等于5名学生的课。

例如,表:

+---------+------------+

| student | class |

+---------+------------+

| A | Math |

| B | English |

| C | Math |

| D | Biology |

| E | Math |

| F | Computer |

| G | Math |

| H | Math |

| I | Math |

+---------+------------+

应该输出:

+---------+

| class |

+---------+

| Math |

+---------+

Note:

学生在每个课中不应被重复计算。

答案1:

select a.class from

(select count(distinct student) as num, class from courses group by class) a

where a.num >= 5

答案2：

select class from courses group by class having count(class) > 4

对比一下效率是一样的：

[SQL] SELECT age from test_user GROUP BY age having count(age)>1;

受影响的行: 0

时间: 0.001s

[SQL]

select a.age from

(select count(age) as num, age from test_user group by age) a

where a.num >= 2;

受影响的行: 0

时间: 0.001s