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。

    取消率的计算方式如下：(被司机或乘客取消的非禁止用户生成的订单数量) / (非禁止用户生成的订单总数)

    +——————+—————————-+
    | Day | Cancellation Rate |
    +——————+—————————-+
    | 2013-10-01 | 0.33 |
    | 2013-10-02 | 0.00 |
    | 2013-10-03 | 0.50 |
    +——————+—————————-+
    答案1
    select tmp2.day2 as Day,ifnull(round(tmp3.count1/tmp2.count2,2),0)as Cancellation_Rate from
    (select count() as count1,tmp1.Request_at as day from (select tr.,us.Banned as DriverBanned ,us2.Banned as ClientBanned from Trips tr join Users us on tr.Driver_Id =us.Users_Id join Users us2 on tr.Client_Id =us2.Users_Id where us.Banned=’NO’ AND us2.Banned=’NO’ AND tr.Status in (‘cancelled_by_driver’,’cancelled_by_client’)) tmp1 group by tmp1.Request_at) tmp3 right join (SELECT COUNT() as count2, tmp4.Request_at as day2 FROM
    (select tr.    ,us.Banned as DriverBanned ,us2.Banned as ClientBanned from Trips tr join Users us on tr.Driver_Id =us.Users_Id join Users us2 on tr.Client_Id =us2.Users_Id where us.Banned=’NO’ AND us2.Banned=’NO’)tmp4 group by tmp4.Request_at) tmp2 on tmp3.day =tmp2.day2 略复杂