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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
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