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LeetCode Database Trips and Users 262

262. Trips and Users
  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 clients 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        |
+------------+-------------------+

大体意思

写一个 SQL 语句，查询非禁止客户（Users 表中 Banned 列为 No 的客户）在 2013-10-1 至 2013-10-3 间的单据取消率，结果为四舍五入后的两位有效数字。

别人的解法

SELECT Request_at DAY,
       ROUND(SUM(IF(Status = 'completed', 0, 1)) / COUNT(*), 2) 'Cancellation Rate'
FROM   Trips t
	LEFT JOIN Users t1 ON t.Client_Id = t1.Users_Id
WHERE  t1.Banned = 'No' AND Request_at BETWEEN '2013-10-01' AND '2013-10-03'
GROUP  BY t.Re	quest_at;

这道题主要是统计计算

笔记：
1、函数：ROUND(column_name,decimals)
column_name 必需。要舍入的字段
decimals 必需。规定要返回的小数位数

2、SUM(IF(Status = 'completed', 0, 1))
统计 SUM 这里也好巧妙

3、having 与 where 的区别
having 字句可以让我们筛选成组后的各种数据，where 字句在聚合前先筛选记录，也就是说作用在 group by 和 having 字句前。而 having 子句在聚合后对组记录进行筛选。

SELECT region, SUM(population), SUM(area)
FROM bbc
GROUP BY region
HAVING SUM(area) > 1000000

在这里，我们不能用 where 来筛选超过 1000000 的地区，因为表中不存在这样一条记录。相反，having 子句可以让我们筛选成组后的各组数据

Reference:
LeetCode：Trips and Users - 出租车接单取消率 - Tsybius2014 > mysql 中的 where 和 having 子句的区别 - Hukin