题解 | #统计作答次数#

# 1.单表查询，关键在于对题意的理解
# total_pv计算：count(start_time)
# complete_pv计算：count(submit_time)
# 从submit_time is not null 的试卷中计算:complete_exam_cnt: distinct count(exam_id) 是错的,因为这样会把没有完成的试卷也算上，所以需要用到case when进行筛选。
# 还有一个方法，就是加一个子查询来筛选出submit_time is not null 的exam_id

select count(start_time) as total_pv,
count(submit_time) as complete_pv,
count(distinct case when submit_time is not null then exam_id
else null end) as complete_exam_cnt
from exam_record