题解 | #每个题目和每份试卷被作答的人数和次数#

(SELECT exam_id AS test_id,
    COUNT(DISTINCT uid) AS uv,
    COUNT(exam_id) AS pv
FROM exam_record
GROUP BY exam_id
ORDER BY test_id, uv, pv)
UNION ALL
(SELECT question_id AS test_id,
    COUNT(DISTINCT uid) AS uv,
    COUNT(question_id) AS pv
FROM practice_record
GROUP BY question_id)
ORDER BY LEFT(test_id,1) DESC, uv DESC, pv DESC;