题解 | #统计每个学校的答过题的用户的平均答题数#

SELECT ei.exam_id, COUNT(distinct ui.uid) uv,
ROUND(SUM(score)/COUNT(score),1) avg_score
FROM examination_info ei 
JOIN exam_record er ON ei.exam_id=er.exam_id 
JOIN user_info ui ON ui.uid=er.uid
WHERE tag='SQL' AND level>5 
    			AND left(er.start_time,9)= left(ei.release_time,9)
GROUP BY ei.exam_id
ORDER BY uv desc, avg_score