题解 | 作答试卷得分大于过80的人的用户等级分布

# 步骤分析：
  # 作答SQL试卷的用户
  # 得分在80分以上
  # 用户等级分组统计人数，降序排序
SELECT u.level, COUNT(DISTINCT er.uid) AS level_cnt
FROM exam_record AS er
LEFT OUTER JOIN examination_info AS ei
ON er.exam_id = ei.exam_id
LEFT OUTER JOIN user_info AS u
ON er.uid = u.uid
WHERE ei.tag = 'SQL'
AND er.score > 80
GROUP BY u.level
ORDER BY level_cnt DESC, level DESC;