LC 1280学生们参加各科测试的次数Students and Examinations　简单

Students、Subjects、Examinations 三表；对每位学生与每门科目的组合统计参加测试的次数（没参加为 0），按学生 id、科目名排序。

思路 Students CROSS JOIN Subjects 补齐全组合，再 LEFT JOIN 计数。

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

数据表：Students,Subjects,Examinations　·　关键字：对每位学生与每门科目的组合统计参加测试的次数（没参加为 0），按学生 id、科目名排序