能帮忙看下为什么三张表直接连不可以，exam_id是错的 select er.uid, count( distinct(case when year(er.submit_time)='2021'and er.submit_time is not null then er.exam_id else null end)) as exam_cnt, count(distinct (case when year(pr.submit_time)='2021' then pr.id else null end)) as question_cnt from exam_record as er join examination_info as ei using(exam_id) left join practice_record as pr using(uid) left join user_info as ui using(uid) where ei.difficulty='hard' and ei.tag="SQL" and ui.level=7 group by er.uid having avg(er.score) >80 order by exam_cnt asc,question_cnt desc