题解 | #0级用户高难度试卷的平均用时和平均得分#

SELECT 
  ui.uid,
  round(AVG(COALESCE(er.score, 0)),0) AS avg_score,
  round(AVG(COALESCE(TIMESTAMPDIFF(minute, er.start_time, er.submit_time), duration)),1) AS avg_time_took
FROM 
  user_info ui
LEFT JOIN 
  exam_record er ON ui.uid = er.uid
LEFT JOIN 
  examination_info ei ON er.exam_id = ei.exam_id
WHERE 
  ei.difficulty = 'hard' and ui.level=0
GROUP BY 
  ui.uid;

注意一下COALESCE的用法，别舍近求远了。写 if写得累死了