LeetCode in Kotlin
1280. Students and Examinations
Easy
SQL Schema
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 the 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.
Return the result table ordered by student_id and subject_name.
The query result format is in the following example.
Example 1:
Input: 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 |
+------------+--------------+
Output:
+------------+--------------+--------------+----------------+
| 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 |
+------------+--------------+--------------+----------------+
Explanation:
The result table should contain all students and all subjects.
Alice attended the Math exam 3 times, the Physics exam 2 times, and the Programming exam 1 time.
Bob attended the Math exam 1 time, the Programming exam 1 time, and did not attend the Physics exam.
Alex did not attend any exams.
John attended the Math exam 1 time, the Physics exam 1 time, and the Programming exam 1 time.
Solution
# Write your MySQL query statement below
SELECT s.student_id, s.student_name, sub.subject_name,
sum(case when sub.subject_name = e.subject_name then 1 else 0 end) as attended_exams
FROM Students s
cross join subjects sub
left join examinations e on e.student_id=s.student_id
group by 1,2,3
order by 1,2,3