明确题意：

统计有未完成状态的试卷的未完成数incomplete_cnt和未完成率incomplete_rate。

问题分解：

• 统计每次作答是否未完成，生成子表 t_exam_record_complete:
  • 计算完成状态：IF(score IS NULL, 1, 0) as is_incomplete
• 按试卷分组：GROUP BY exam_id
• 计算每张试卷的未完成数和未完成率：
  • 未完成数：SUM(is_incomplete) as incomplete_cnt
  • 未完成率：AVG(is_incomplete) as incomplete_rate
  • 保留3位小数：ROUND(x, 3)
• 筛选有未完成的分组（试卷）：HAVING incomplete_cnt > 0

细节问题：

• 表头重命名：as

完整代码：

SELECT exam_id, SUM(is_incomplete) as incomplete_cnt,
    round(AVG(is_incomplete), 3) as incomplete_rate
FROM (
    SELECT exam_id, IF(score IS NULL, 1, 0) as is_incomplete
    FROM exam_record
) as t_exam_record_complete
GROUP BY exam_id
HAVING incomplete_cnt > 0