LeetCode Database 262 行程和用户

1. 题目描述

Trips 表中存所有出租车的行程信息。每段行程有唯一键 Id，Client_Id 和 Driver_Id 是 Users 表中 Users_Id 的外键。Status 是枚举类型，枚举成员为 (‘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 表存所有用户。每个用户有唯一键 Users_Id。Banned 表示这个用户是否被禁止，Role 则是一个表示（‘client’, ‘driver’, ‘partner’）的枚举类型。

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

写一段 SQL 语句查出 2013年10月1日 至 2013年10月3日 期间非禁止用户的取消率。基于上表，你的 SQL 语句应返回如下结果，取消率（Cancellation Rate）保留两位小数。

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

2. 题解

2.1. MySQL

取消率由Status字段决定。Status字段的值可以为completed、cancelled_by_driver、cancelled_by_client，只有completed代表不取消，其它两个值就代表取消。那么取消率就是cancelled_by_driver和cancelled_by_client所占的比例

这里可以用CASE语句来统计，只有completed状态视为0，其它状态视为1，累加起来，最除以总数COUNT(*)，就是取消率

CASE 
    WHEN tb1.Status = 'completed' THEN 0
    ELSE 1
END

ROUND(value, n) 表示对value四舍五入保留N位小数

-- 正确做法
SELECT tb1.Request_at AS Day, (
    ROUND(SUM(CASE WHEN tb1.Status = 'completed' THEN 0
    ELSE 1
    END) / COUNT(*), 2)) AS 'Cancellation Rate'
FROM Trips tb1
JOIN Users tb2
ON tb1.Client_Id = tb2.Users_Id AND tb2.Banned = 'No'  -- Client是非禁止用户
JOIN Users tb3
ON tb1.Driver_id = tb3.Users_Id AND tb3.Banned = 'No'  -- Driver是非禁止用户
WHERE tb1.Request_at BETWEEN '2013-10-01' AND '2013-10-03'  -- 2013年10月1日 至 2013年10月3日
GROUP BY tb1.Request_at  -- 分组统计
ORDER BY Day             -- 日期从小到大