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

step2：按照city_name分组，统计不重复司机的个数；

step3：显示司机数大于15的城市名称和司机数；

step4：按司机数排序

SELECT b.city_name, COUNT(DISTINCT a.driver_id) as driver_num

FROM driver_tb a

LEFT JOIN city_tb b

ON a.city_id=b.city_id

GROUP BY b.city_name

HAVING COUNT(DISTINCT a.driver_id)>15

ORDER BY driver_num;