去重计数 distinct ，条件查询where select  count(distinct device_id) did_cnt, count(question_id) question_cnt  from  question_practice_detail  where  date_format(date,"%Y-%m")='2021-08';

select   u.university,   qd.difficult_level,   round(     count(q.question_id) / count(distinct q.device_id),     4   ) avg_answer_cnt from   ...

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