题解 | #统计作答次数#
统计作答次数
http://www.nowcoder.com/practice/45a87639110841b6950ef6a12d20175f
count典型题目
1、count(*) 不论null都进行统计； 2、count(1) 不论null都进行统计； 3、count(字段) 只统计不为 null的数据 4、count(distinct ) 去重统计
SELECT
count(*) 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
