题目

SQL 架构

Create table If Not Exists courses (student varchar(255), class varchar(255))
Truncate table courses
insert into courses (student, class) values ('A', 'Math')
insert into courses (student, class) values ('B', 'English')
insert into courses (student, class) values ('C', 'Math')
insert into courses (student, class) values ('D', 'Biology')
insert into courses (student, class) values ('E', 'Math')
insert into courses (student, class) values ('F', 'Computer')
insert into courses (student, class) values ('G', 'Math')
insert into courses (student, class) values ('H', 'Math')
insert into courses (student, class) values ('I', 'Math')

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。

题解

将表按 class 分组，当单个分组人数大于等于 5 时输出，不过也许有重复的记录，所以还需要去重

SELECT DISTINCT class 
FROM courses 
GROUP BY class
HAVING COUNT(DISTINCT student)>=5;