SELECT IFNULL(t.max, 0) as max
FROM (
       SELECT max(x.total) as max
       FROM (
              SELECT @total := 0 as total,
                     @start := UNIX_TIMESTAMP('2019-01-23 10:30:00 AM'),
                     @end := UNIX_TIMESTAMP('2019-01-23 12:00:00 PM')
              UNION
              SELECT @total := @total + qty as total, null, null
              FROM (
                     SELECT start_time as time, qty
                     FROM temp_events
                     WHERE end_time >= @start
                       AND start_time <= @end
                     UNION ALL
                     SELECT end_time as time, -qty as qty
                     FROM temp_events
                     WHERE end_time >= @start
                       AND start_time <= @end
                     ORDER BY time ASC, qty ASC
                   ) as y) x) t