官方题解 | #某滴出行9月各城市每天的司机数#
某滴出行9月各城市每天的司机数
http://www.nowcoder.com/practice/3487caa8501c4490bbbff25620b88781
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;
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