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1280. 学生们参加各科测试的次数 - 力扣（LeetCode）

学生表: Students

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| student_id    | int     |
| student_name  | varchar |
+---------------+---------+
在 SQL 中，主键为 student_id（学生ID）。
该表内的每一行都记录有学校一名学生的信息。

科目表: Subjects

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| subject_name | varchar |
+--------------+---------+
在 SQL 中，主键为 subject_name（科目名称）。
每一行记录学校的一门科目名称。

考试表: Examinations

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| student_id   | int     |
| subject_name | varchar |
+--------------+---------+
这个表可能包含重复数据（换句话说，在 SQL 中，这个表没有主键）。
学生表里的一个学生修读科目表里的每一门科目。
这张考试表的每一行记录就表示学生表里的某个学生参加了一次科目表里某门科目的测试。

查询出每个学生参加每一门科目测试的次数，结果按 student_id 和 subject_name 排序。

查询结构格式如下所示。

示例 1：

输入：
Students table:
+------------+--------------+
| student_id | student_name |
+------------+--------------+
| 1          | Alice        |
| 2          | Bob          |
| 13         | John         |
| 6          | Alex         |
+------------+--------------+
Subjects table:
+--------------+
| subject_name |
+--------------+
| Math         |
| Physics      |
| Programming  |
+--------------+
Examinations table:
+------------+--------------+
| student_id | subject_name |
+------------+--------------+
| 1          | Math         |
| 1          | Physics      |
| 1          | Programming  |
| 2          | Programming  |
| 1          | Physics      |
| 1          | Math         |
| 13         | Math         |
| 13         | Programming  |
| 13         | Physics      |
| 2          | Math         |
| 1          | Math         |
+------------+--------------+
输出：
+------------+--------------+--------------+----------------+
| student_id | student_name | subject_name | attended_exams |
+------------+--------------+--------------+----------------+
| 1          | Alice        | Math         | 3              |
| 1          | Alice        | Physics      | 2              |
| 1          | Alice        | Programming  | 1              |
| 2          | Bob          | Math         | 1              |
| 2          | Bob          | Physics      | 0              |
| 2          | Bob          | Programming  | 1              |
| 6          | Alex         | Math         | 0              |
| 6          | Alex         | Physics      | 0              |
| 6          | Alex         | Programming  | 0              |
| 13         | John         | Math         | 1              |
| 13         | John         | Physics      | 1              |
| 13         | John         | Programming  | 1              |
+------------+--------------+--------------+----------------+
解释：
结果表需包含所有学生和所有科目（即便测试次数为0）：
Alice 参加了 3 次数学测试, 2 次物理测试，以及 1 次编程测试；
Bob 参加了 1 次数学测试, 1 次编程测试，没有参加物理测试；
Alex 啥测试都没参加；
John  参加了数学、物理、编程测试各 1 次。

Create table If Not Exists Students (student_id int, student_name varchar(20))
Create table If Not Exists Subjects (subject_name varchar(20))
Create table If Not Exists Examinations (student_id int, subject_name varchar(20))
Truncate table Students
insert into Students (student_id, student_name) values ('1', 'Alice')
insert into Students (student_id, student_name) values ('2', 'Bob')
insert into Students (student_id, student_name) values ('13', 'John')
insert into Students (student_id, student_name) values ('6', 'Alex')
Truncate table Subjects
insert into Subjects (subject_name) values ('Math')
insert into Subjects (subject_name) values ('Physics')
insert into Subjects (subject_name) values ('Programming')
Truncate table Examinations
insert into Examinations (student_id, subject_name) values ('1', 'Math')
insert into Examinations (student_id, subject_name) values ('1', 'Physics')
insert into Examinations (student_id, subject_name) values ('1', 'Programming')
insert into Examinations (student_id, subject_name) values ('2', 'Programming')
insert into Examinations (student_id, subject_name) values ('1', 'Physics')
insert into Examinations (student_id, subject_name) values ('1', 'Math')
insert into Examinations (student_id, subject_name) values ('13', 'Math')
insert into Examinations (student_id, subject_name) values ('13', 'Programming')
insert into Examinations (student_id, subject_name) values ('13', 'Physics')
insert into Examinations (student_id, subject_name) values ('2', 'Math')
insert into Examinations (student_id, subject_name) values ('1', 'Math')

 

SELECT

    s.student_id,

s.student_name,

sub.subject_name,

IFNULL(grouped.attended_exams, 0) AS attended_exams

FROM

    Students s

CROSS JOIN

    Subjects sub

LEFT JOIN (

    SELECT student_id, subject_name, COUNT(*) AS attended_exams

    FROM Examinations

    GROUP BY student_id, subject_name

) grouped

ON s.student_id = grouped.student_id AND sub.subject_name = grouped.subject_name

ORDER BY s.student_id, sub.subject_name;