1280. Students and Examinations

https://leetcode.com/problems/students-and-examinations

Description

Table: Students

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| student\_id    | int     |
| student\_name  | varchar |
+---------------+---------+
student\_id is the primary key for this table.
Each row of this table contains the ID and the name of one student in the school.

Table: Subjects

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| subject\_name | varchar |
+--------------+---------+
subject\_name is the primary key for this table.
Each row of this table contains the name of one subject in the school.

Table: Examinations

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| student\_id   | int     |
| subject\_name | varchar |
+--------------+---------+
There is no primary key for this table. It may contain duplicates.
Each student from the Students table takes every course from Subjects table.
Each row of this table indicates that a student with ID student\_id attended the exam of subject\_name.

Write an SQL query to find the number of times each student attended each exam.

Order the result table by student_id and subject_name.

The query result format is in the following example:

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         |
+------------+--------------+
Result table:
+------------+--------------+--------------+----------------+
| 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              |
+------------+--------------+--------------+----------------+
The result table should contain all students and all subjects.
Alice attended Math exam 3 times, Physics exam 2 times and Programming exam 1 time.
Bob attended Math exam 1 time, Programming exam 1 time and didn't attend the Physics exam.
Alex didn't attend any exam.
John attended Math exam 1 time, Physics exam 1 time and Programming exam 1 time.

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