Mysql – Calculate MSB on MySQL

if i understand you correctly, here will do:

select user, sum(RT) 
from test2 a
inner join ( select distinct(time) from test2 order by time desc limit 2) t
on a.time=t.time
group by user

The 'N' for number of latest batch is determined by the limit 2 in the subquery t.

For example, if i run the above query, showing data for the last 2 batches, the output is:

Dailymotion 8.11
Facebook    1.44
Google+     0.98
Instagram   0.15
Metacafe    0.80
Twitter     0.75
Youtube     0.48

But the query above won't show any users who don't have data in the last 'N' batch. If you need to show those users as well (as '0' RT), you will have to join the above query with your users table.

P/S: The test2 table in the query above contains the exact data as in your first table in your post.

Before MySQL 5.6.3, if a statement that generates a warning or error causes a condition handler to be invoked, the handler may not clear the diagnostic area. This might lead to the appearance that the handler was not invoked.

I have explained this kind of behavior and the solution in this post:

Event Scheduler: No data - zero rows fetched, selected, or processed