如果用户APPS在一台设备上使用多个，则该查询将使用APPS具有最高值的行usage_start_time：
select a.userid
, sum(u.high_count)
, sum(u.medium_count)
from apps a
join `usage` u
on u.appid = a.id
join (
select u.device_id
, a.userid
, max(a.usage_start_time) as max_start_time
from apps a
join `usage` u
on u.appid = a.id
group by
u.device_id
, a.userid
) filter
on filter.device_id = u.device_id
and filter.userid = a.userid
and filter.max_start_time = a.usage_start_time
group by
a.userid
在您的数据集中，它将5, 3, 4为user选择使用情况行12。
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