SQL例题求已完成的试卷数complete_exam_cnt---表子查询结果也可以作为select里的选择项

 

法一：

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

法二：

SELECT
    count(distinct exam_id and score is not null) as complete_exam_cnt#将试卷去重，同时将筛选完成的试卷
from exam_record;

法三：

select
    count(distinct case when score is not null then exam_id else null end) as complete_exam_cnt
from exam_record ;