题解 | #统计作答次数#

select 
    count(uid) as total_pv
    ,count(score) as complete_pv
    ,count(distinct if(score is null,null,exam_id)) as complete_exam_cnt
from exam_record

主要是在select中去过滤其他字段

如果score为null，那么该行的exam_id就为null，不会被统计到count里

select count(distinct if(score is null,null,exam_id)) as complete_exam_cnt