官方题解 | #滴滴所载的乘客数大于1的司机id，乘客数以及所在的城市名称#
滴滴所载的乘客数大于1的司机id，乘客数以及所在的城市名称
http://www.nowcoder.com/practice/5ad43d3f2bc144d08111b10b9cf06683
step1：使用driver_id相等的条件，将order_info_tb左连接driver_tb；
step2：使用city_id相等的条件，左连接city_tb；
step3：按照driver_id分组，统计不重复的乘客数；
step4：只显示乘客数大于1的相关信息；
step5：按司机编号排序。
select driver_id, COUNT(DISTINCT passenger_id) as passenger_num, city_name
FROM(
SELECT a.driver_id, city_name, passenger_id FROM order_info_tb a
LEFT JOIN driver_tb b
ON a.driver_id=b.driver_id
LEFT JOIN city_tb c
ON b.city_id=c.city_id)t
GROUP BY driver_id
HAVING passenger_num>1
ORDER BY driver_id;
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