order_id | time_order
1 | 2018-10-10 00:00:00
2 | 2018-10-11 01:01:01
4 | 2018-10-12 02:02:02
5 | 2018-10-12 03:03:03
6 | 2018-10-13 04:04:04
---
7 | 2018-10-14 05:05:05
...

SELECT DATE(time_order), COUNT(*)
FROM orders
WHERE DATE(time_order) >= DATE(NOW()) - INTERVAL 7 DAY
GROUP BY DATE(time_order);