明确题意：

统计作答SQL类别的试卷得分大于过80的人的用户等级分布，按数量降序排序

问题分解：

• 找到每个满足条件的用户及他们的等级，生成子表t_ml_user_level：
  • 内连接试卷作答表和用户信息表：exam_record JOIN user_info using(uid)
  • 筛选作答SQL类别的试卷得分大于过80的人：
    • 筛选分数大于80：WHERE score > 80
    • 筛选试卷类别为SQL：exam_id IN (SELECT exam_id FROM examination_info WHERE tag = 'SQL')
• 按用户等级分组：GROUP BY level
• 统计各等级数量：count( uid ) AS level_cnt

细节问题：

• 表头重命名：as
• 按数量降序排序：ORDER BY level_cnt DESC

完整代码：

SELECT level, count( uid ) AS level_cnt 
FROM (
    SELECT DISTINCT exam_record.uid AS uid, level 
    FROM exam_record
    JOIN user_info using(uid) 
    WHERE score > 80 AND exam_id IN ( 
        SELECT exam_id FROM examination_info WHERE tag = 'SQL' 
    ) 
) AS t_ml_user_level 
GROUP BY level 
ORDER BY level_cnt DESC;