LeetCode（数据库）- 学生们参加各科测试的次数

Posted 2021-09-30 程序员牧码

题目链接：点击打开链接

题目大意：略。

解题思路：略。

AC 代码

-- 解决方案(1)
select
ss.student_id as student_id,
ss.student_name as student_name,
ss.subject_name as subject_name,
ifnull(e1.attended_exams, 0) as attended_exams
from
(
    select
    *
    from `Students` as s1 
    cross join `Subjects` as s2
) as ss
left join
(
    select
    student_id, 
    subject_name,
    count(student_id) as attended_exams
    from `Examinations`
    group by student_id,subject_name
) as e1
on ss.student_id = e1.student_id
and ss.subject_name = e1.subject_name
order by ss.student_id, ss.subject_name

-- 解决方案(2)
SELECT s.student_id, student_name, sub.subject_name, COUNT(e.subject_name) attended_exams 
FROM Subjects sub JOIN Students s LEFT JOIN Examinations e ON e.subject_name = sub.subject_name AND e.student_id = s.student_id
GROUP BY s.student_id, sub.subject_name
ORDER BY s.student_id, sub.subject_name