SELECT* FROM ( SELECT exam_id,COUNT(DISTINCT uid) AS uv,COUNT(exam_id) AS pv FROM exam_record GROUP BY exam_id ORDER BY uv DESC,pv DESC) t1 UNION ALL SELECT * FROM ( SELECT question_id,COUNT(DISTINCT uid) AS uv,COUNT(question_id) AS pv FROM practice_record GROUP BY question_id ORDER BY uv DESC,pv DESC ) t2;

切记，ORDER BY 后面不能直接跟UNION ，但可以在子查询中进行使用