+-----------+----------------+-------------+------+
| adnest_id | visitor_id     | timeslot_id | num  |
+-----------+----------------+-------------+------+
|        89 |  9924542981379 |        6058 |    1 |
|        89 | 12141306723875 |        6022 |    1 |
|        89 | 12141306723875 |        6044 |    2 |
|        89 | 12141306723875 |        6064 |    2 |
+-----------+----------------+-------------+------+

+-----------+----------------+-----------+
| adnest_id | visitor_id     | frequency |
+-----------+----------------+-----------+
|        89 |  9924542981379 |         1 |
|        89 | 12141306723875 |         3 |
+-----------+----------------+-----------+

SELECT adnest_id, visitor_id, count(visitor_id) as frequency
FROM hist_impressions
INNER JOIN timeslot t ON t.timeslot_id = hist_impressions.timeslot_id
WHERE start_date BETWEEN @start AND ADDDATE(@start, INTERVAL 7 DAY)
group by visitor_id
limit 10;