LeetCode.596. 超過5名學生的課

題目

SQL 架構

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。

題解

將表按 class 分組，當單個分組人數大於等於 5 時輸出，不過也許有重複的記錄，所以還需要去重

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