Easy Leetcode problem ~ StudentsAndExaminations.sql

LeetCode/SQL50/StudentsAndExaminations.sql

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+-- Table: Students
+
+-- +---------------+---------+
+-- | Column Name   | Type    |
+-- +---------------+---------+
+-- | student_id    | int     |
+-- | student_name  | varchar |
+-- +---------------+---------+
+-- student_id is the primary key (column with unique values) 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 (column with unique values) 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 (column with unique values) 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 a solution to find the number of times each student attended each exam.
+
+-- Return the result table ordered by student_id and subject_name.
+
+-- The 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 in PostgreSQL
+with combined_dataset as
+(
+    select s.student_id,
+        s.student_name,
+        sub.subject_name
+    from students s 
+        cross join subjects sub
+)
+select cd.student_id,
+       cd.student_name,
+       cd.subject_name,
+       case when ae.attended_exams is null then 0
+            else ae.attended_exams
+        end as attended_exams
+from combined_dataset cd
+left join
+(
+    select student_id,
+           subject_name,
+           count(*) as attended_exams
+    from Examinations 
+    group by student_id,
+            subject_name
+) ae
+on (cd.student_id = ae.student_id
+   and cd.subject_name = ae.subject_name)
+order by cd.student_id,cd.subject_name;