262. Trips and Users

Description

Difficulty: Hard
Tag: Sql

The Trips table holds all taxi trips. Each trip has a unique Id, while Client_Id and Driver_Id are both foreign keys to the Users_Id at the Users table. Status is an ENUM type of (‘completed’, ‘cancelled_by_driver’, ‘cancelled_by_client’).

+----+-----------+-----------+---------+--------------------+----------+
| Id | Client_Id | Driver_Id | City_Id |        Status      |Request_at|
+----+-----------+-----------+---------+--------------------+----------+
| 1  |     1     |    10     |    1    |     completed      |2013-10-01|
| 2  |     2     |    11     |    1    | cancelled_by_driver|2013-10-01|
| 3  |     3     |    12     |    6    |     completed      |2013-10-01|
| 4  |     4     |    13     |    6    | cancelled_by_client|2013-10-01|
| 5  |     1     |    10     |    1    |     completed      |2013-10-02|
| 6  |     2     |    11     |    6    |     completed      |2013-10-02|
| 7  |     3     |    12     |    6    |     completed      |2013-10-02|
| 8  |     2     |    12     |    12   |     completed      |2013-10-03|
| 9  |     3     |    10     |    12   |     completed      |2013-10-03| 
| 10 |     4     |    13     |    12   | cancelled_by_driver|2013-10-03|
+----+-----------+-----------+---------+--------------------+----------+

The Users table holds all users. Each user has an unique Users_Id, and Role is an ENUM type of (‘client’, ‘driver’, ‘partner’).

+----------+--------+--------+
| Users_Id | Banned |  Role  |
+----------+--------+--------+
|    1     |   No   | client |
|    2     |   Yes  | client |
|    3     |   No   | client |
|    4     |   No   | client |
|    10    |   No   | driver |
|    11    |   No   | driver |
|    12    |   No   | driver |
|    13    |   No   | driver |
+----------+--------+--------+

Write a SQL query to find the cancellation rate of requests made by unbanned users between Oct 1, 2013 and Oct 3, 2013. For the above tables, your SQL query should return the following rows with the cancellation rate being rounded to two decimal places.

+------------+-------------------+
|     Day    | Cancellation Rate |
+------------+-------------------+
| 2013-10-01 |       0.33        |
| 2013-10-02 |       0.00        |
| 2013-10-03 |       0.50        |
+------------+-------------------+

Solution

201ms:

select a.Request_at Day, round(sum(case when a.status = 'cancelled_by_driver' then 1 when a.status = 'cancelled_by_client' then 1 else 0 end) /count(1), 2) as 'Cancellation Rate'
from trips a 
where exists (select 1 from users u where u.users_id = a.client_id and u.banned='No')
and exists (select 1 from users u where u.users_id = a.driver_id and u.banned='No')
and a.Request_at between '2013-10-01' and '2013-10-03'
group by a.Request_at

198ms:

select a.Request_at Day, round(sum(case when a.status != 'completed' then 1  else 0 end) /count(1), 2) as 'Cancellation Rate'
from trips a 
where exists (select 1 from users u where u.users_id = a.client_id and u.banned='No')
and exists (select 1 from users u where u.users_id = a.driver_id and u.banned='No')
and a.Request_at between '2013-10-01' and '2013-10-03'
group by a.Request_at

178ms:

select a.Request_at Day, round(sum(case when a.status = 'cancelled_by_driver' then 1 when a.status = 'cancelled_by_client' then 1 else 0 end) /count(1), 2) as 'Cancellation Rate'
from trips a 
left join users d on d.users_id = a.client_id
left join users c on c.users_id = a.client_id
where d.banned='No' and c.banned = 'No'
and a.Request_at between '2013-10-01' and '2013-10-03'
group by a.Request_at

170ms:

select a.Request_at Day, round(sum(case when a.status != 'completed' then 1 else 0 end) /count(1), 2) as 'Cancellation Rate'
from trips a 
left join users d on d.users_id = a.client_id
left join users c on c.users_id = a.client_id
where d.banned='No' and c.banned = 'No'
and a.Request_at between '2013-10-01' and '2013-10-03'
group by a.Request_at

从作这道题了解到了一个知识点，在 left join 中，最外层的 where 条件是施加在 (from, left join) 完成以后的表上的，而不是仅仅影响 left join 中的表。

若是只想要影响要left join 的表，有两种办法

1. 条件加在 on 后面
2. 临时表，形式为left join (select * fro users u where u.banned = 'No')