我做错了什么？ SQL

Question

我正在尝试生成一份报告，显示每个学生参加每个科目考试的次数，包括出勤率为零的学生和科目。我有三张桌子：

Students

、

Subjects

和

Examinations

。

输入：

Students

表：

+------------+--------------+
| student_id | student_name |
+------------+--------------+
| 1          | Alice        |
| 2          | Bob          |
| 13         | John         |
| 6          | Alex         |
+------------+--------------+

Subjects

表：

+--------------+
| subject_name |
+--------------+
| Math         |
| Physics      |
| Programming  |
+--------------+

Examinations

表：

+------------+--------------+
| 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              |
+------------+--------------+--------------+----------------+

说明： 结果表应包含所有学生和所有科目。 Alice 参加了 3 次数学考试，2 次物理考试，1 次编程考试。 Bob 参加了数学考试 1 次，编程考试 1 次，没有参加物理考试。 亚历克斯没有参加任何考试。 John 参加了数学考试 1 次、物理考试 1 次、编程考试 1 次。

这是我的代码：

SELECT s1.student_id, s1.student_name, e.subject_name, COUNT(e.subject_name) AS attended_exams
FROM Students s1
JOIN Examinations e ON e.student_id = s1.student_id
LEFT JOIN Subjects s2 ON s2.subject_name = e.subject_name
GROUP BY s1.student_id, s1.student_name, e.subject_name
ORDER BY s1.student_id ASC, s1.student_name ASC

这是我的输出：

+------------+--------------+---------------+----------------+
| 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          | Programming   |              1 |
|         13 | John         | Math          |              1 |
|         13 | John         | Physics       |              1 |
|         13 | John         | Programming   |              1 |
+------------+--------------+---------------+----------------+

我仔细检查了解决方案，但没有发现任何差异。有人可以帮我修复我的代码以获得预期的输出吗？

Answer 1

使用

cross join

代替

left join

SELECT s.student_id, s.student_name, sub.subject_name,
       COALESCE(COUNT(e.student_id), 0) AS attended_exams
FROM Students s
CROSS JOIN Subjects sub
LEFT JOIN Examinations e ON e.student_id = s.student_id
                         AND e.subject_name = sub.subject_name
GROUP BY s.student_id, s.student_name, sub.subject_name
ORDER BY s.student_id ASC, sub.subject_name ASC;