明确题意：

统计出总作答次数total_pv、试卷已完成作答数complete_pv、已完成的试卷数complete_exam_cnt

问题分解：

  • 总作答次数：count(exam_id) as total_pv；
  • 试卷已完成作答数，count(A)会忽略A的值为null的行：count(submit_time) as complete_pv；
  • 已完成的试卷数，已完成时才计数用if判断，试卷可能被完成多次，需要去重用distinct：count(distinct if(submit_time is not null, exam_id, null)) as complete_exam_cnt

完整代码：

select
    count(exam_id) as total_pv,
    count(submit_time) as complete_pv,
    count(distinct if(submit_time is not null, exam_id, null)) as complete_exam_cnt
from exam_record