task: #1280

Author: Ivanbyone

README.md

@@ -56,6 +56,7 @@ Have a good contributing!
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     - [1211. Queries Quality and Percentage](./leetcode/easy/1211.%20Queries%20Quality%20and%20Percentage.sql)
     - [1251. Average Selling Price](./leetcode/easy/1251.%20Average%20Selling%20Price.sql)
+    - [1280. Students and Examinations](./leetcode/easy/1280.%20Students%20and%20Examinations.sql)
     - [1327. List the Products Ordered in a Period](./leetcode/easy/1327.%20List%20the%20Products%20Ordered%20in%20a%20Period.sql)
     - [1378. Replace Employee ID With The Unique Identifier](./leetcode/easy/1378.%20Replace%20Employee%20ID%20With%20The%20Unique%20Identifier.sql)
     - [1407. Top Travellers](./leetcode/easy/1407.%20Top%20Travellers.sql)

leetcode/easy/1280. Students and Examinations.sql

@@ -0,0 +1,59 @@
+/*
+Question 1280. Students and Examinations
+Link: https://leetcode.com/problems/students-and-examinations/description/?envType=study-plan-v2&envId=top-sql-50
+
+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.
+*/
+
+SELECT
+    st.student_id,
+    st.student_name,
+    s.subject_name,
+    COUNT(e.student_id) AS attended_exams
+FROM Students AS st
+CROSS JOIN Subjects AS s
+LEFT JOIN
+    Examinations AS e
+    ON
+        st.student_id = e.student_id
+        AND s.subject_name = e.subject_name
+GROUP BY st.student_id, st.student_name, s.subject_name
+ORDER BY st.student_id, st.student_name, s.subject_name