LeetCode - 1280. Students and Examinations (MySQL)

Easy, SQL - LEFT JOIN

RunTime : 970 ms

문제

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.

풀이

• Easy보단 Medium에 가깝지 않나...
• 3개의 테이블을 다중 조인하고 그 중, LEFT JOIN을 통해 없는 결과값에 대해 0을 가져와야 했다.

SELECT S.student_id, S.student_name, U.subject_name, COUNT(E.student_id) AS attended_exams
FROM Students AS S JOIN Subjects AS U LEFT JOIN Examinations AS E
ON S.student_id = E.student_id AND U.subject_name = E.subject_name
GROUP BY 1, 3
ORDER BY 1, 3