Select only last value using group by at mysql

Use a simple group by id_member, but select:

substring(max(concat(from_unixtime(timestamp),attendance)) from 20) as attendance

This attaches attendance to the timestamp for each row in a group, in order to be able to select the desired timestamp/attendance with max() and then extract just the attendance.

What concat() returns is 19 characters of formatted timestamp (YYYY-mm-dd HH:MM:SS) with the attendance appended starting at character 20; the substring(... from 20) gets just the attendance from the (stringwise) maximum one for the group. You can remove the group by and just

select concat(from_unixtime(timestamp),attendance), timestamp, attendance

to get a better idea of how it uses max to get the right attendance.

SELECT id_branch_channel, id_member, attendance, timestamp, id_member 
FROM (select * from view_event_attendance order by timestamp desc) as whatever
WHERE id_event = 782 
GROUP BY id_event,id_member;

EDIT: This may yield better performance:

SELECT *
FROM (SELECT id_branch_channel, id_member, attendance, timestamp, id_member 
      FROM view_event_attendance 
      WHERE id_event = 782 
      ORDER BY timestamp desc
     ) as whatever
GROUP BY id_event,id_member;

As long as the result-set can fit into the Innodb_buffer_pool, you will not see a significant performance drop.