step1：使用city_id相等的条件，将driver_tb左连接city_tb；

step2：限制日期范围为2021年9月，按照城市和日期分组，统计不重复用户的个数。

select city_name, dt, count(distinct driver_id) as driver_num

FROM driver_tb a

LEFT JOIN city_tb b

ON a.city_id = b.city_id

WHERE dt BETWEEN '2021-9-1' and '2021-9-30'

GROUP BY a.city_id, dt;