为什么这个timestampdiff CASE收益率AVG低于WHEN？

Question

我想通过源报告获取平均卸载日期，我有一个

select i.source,u.avgday from inst i
left join ( select u.app_uid,
  case 
    when timestampdiff(day,i.app_installed_datetime, u.timestamp) > 3
    then timestampdiff(day,i.app_installed_datetime, u.timestamp) 
   else null end as avgday
  from uninst u
  inner join inst i
  on i.app_uid = u.app_uid
) as u
on u.app_uid = i.app_uid
group by i.source

为什么我在0.1-10范围内获得u.avgday结果？难道这只有AVG，其中有一个值，否则它是否为空？

Answer 1

我认为您打算使用AVG()聚合函数，如

  avg(case 
    when timestampdiff(day,i.app_installed_datetime, u.timestamp) > 3
    then timestampdiff(day,i.app_installed_datetime, u.timestamp) 
    end) as avgday