官方题解 | #某滴出行司机数大于15的城市#
某滴出行司机数大于15的城市
http://www.nowcoder.com/practice/7a92013dea444ca4b3a39db76cc52ac9
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;
