262. 行程和用戶

Trips 表中存所有出租車的行程信息。每段行程有唯一健 Id，Client_Id 和 Driver_Id 是 Users 表中 Users_Id 的外鍵。Status 是枚舉類型，枚舉成員爲 (‘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|
+----+-----------+-----------+---------+--------------------+----------+

Users 表存所有用戶。每個用戶有唯一鍵 Users_Id。Banned 表示這個用戶是否被禁止，Role 則是一個表示（‘client’, ‘driver’, ‘partner’）的枚舉類型。

+----------+--------+--------+
| 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 |
+----------+--------+--------+

寫一段 SQL 語句查出 2013年10月1日 至 2013年10月3日 期間非禁止用戶的取消率。基於上表，你的 SQL 語句應返回如下結果，取消率（Cancellation Rate）保留兩位小數。

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

解析：

SELECT Request_at Day, 
ROUND(COUNT(IF(Status != 'completed', TRUE, NULL)) / COUNT(*), 2) 'Cancellation Rate' 
            FROM Trips 
            WHERE (Request_at between '2013-10-01' and '2013-10-03') and Client_Id IN (SELECT Users_Id FROM Users WHERE Banned = 'No') 
            GROUP BY Request_at;

1. 用case when語句把取消訂單數量算出來。
2. users表中banned列值是‘Yes’的用戶id在trips表中司機id列和用戶id列中刨掉。
3. 注意時間列的類型轉換。
4. 注意得出來的比率要用round函數取小數點後兩位

select t.Request_at Day, 
round(sum(case when t.Status like 'cancelled_%' then 1 else 0 end)/count(*),2) as "Cancellation Rate"
            from Trips t 
            inner join Users u 
            on t.Client_Id = u.Users_Id and u.Banned='No'
            where t.Request_at between '2013-10-01' and '2013-10-03'
            group by t.Request_at