1. 题目

支出表: Spending

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| user_id     | int     |
| spend_date  | date    |
| platform    | enum    | 
| amount      | int     |
+-------------+---------+
这张表记录了用户在一个在线购物网站的支出历史，
该在线购物平台同时拥有桌面端（'desktop'）和手机端（'mobile'）的应用程序。
这张表的主键是 (user_id, spend_date, platform)。
平台列 platform 是一种 ENUM ，类型为（'desktop', 'mobile'）。

写一段 SQL 来查找每天 仅 使用手机端用户、仅 使用桌面端用户、 同时 使用桌面端和手机端的用户人数和总支出金额。

查询结果格式如下例所示：

Spending table:
+---------+------------+----------+--------+
| user_id | spend_date | platform | amount |
+---------+------------+----------+--------+
| 1       | 2019-07-01 | mobile   | 100    |
| 1       | 2019-07-01 | desktop  | 100    |
| 2       | 2019-07-01 | mobile   | 100    |
| 2       | 2019-07-02 | mobile   | 100    |
| 3       | 2019-07-01 | desktop  | 100    |
| 3       | 2019-07-02 | desktop  | 100    |
+---------+------------+----------+--------+Result table:
+------------+----------+--------------+-------------+
| spend_date | platform | total_amount | total_users |
+------------+----------+--------------+-------------+
| 2019-07-01 | desktop  | 100          | 1           |
| 2019-07-01 | mobile   | 100          | 1           |
| 2019-07-01 | both     | 200          | 1           |
| 2019-07-02 | desktop  | 100          | 1           |
| 2019-07-02 | mobile   | 100          | 1           |
| 2019-07-02 | both     | 0            | 0           |
+------------+----------+--------------+-------------+ 
在 2019-07-01, 
用户1 同时 使用桌面端和手机端购买, 
用户2 仅 使用了手机端购买，
而用户3 仅 使用了桌面端购买。在 2019-07-02, 
用户2 仅 使用了手机端购买, 
用户3 仅 使用了桌面端购买，
且没有用户 同时 使用桌面端和手机端购买。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/user-purchase-platform
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

• 先造出表的各种组合

select distinct spend_date, "desktop" platform from Spending
union
select distinct spend_date, "mobile" platform from Spending
union
select distinct spend_date, "both" platform from Spending

{"headers": ["spend_date", "platform"], 
"values": [
["2019-07-01", "desktop"], 
["2019-07-02", "desktop"], 
["2019-07-01", "mobile"], 
["2019-07-02", "mobile"], 
["2019-07-01", "both"], 
["2019-07-02", "both"]]}

• 计算每天，某类属下的总金额、人数

select spend_date, if(count(distinct platform)=1, platform, 'both') plat,sum(amount) total_am,count(distinct user_id) total_u # 1 total_u 这么写也对，就1个人
from Spending
group by spend_date, user_id

{"headers": ["spend_date", "plat", "total_am", "total_u"], 
"values": [
["2019-07-01", "both", 200, 1], 
["2019-07-01", "mobile", 100, 1], 
["2019-07-01", "desktop", 100, 1], 
["2019-07-02", "mobile", 100, 1], 
["2019-07-02", "desktop", 100, 1]]}

• 上面2表连接

select p.spend_date, p.platform, t.total_am, t.total_u
from
(select distinct spend_date, "desktop" platform from Spendingunionselect distinct spend_date, "mobile" platform from Spendingunionselect distinct spend_date, "both" platform from Spending
) p
left join
(select spend_date, if(count(distinct platform)=1, platform, 'both') plat,sum(amount) total_am,count(distinct user_id) total_ufrom Spendinggroup by spend_date, user_id
) t
on p.platform = t.plat and p.spend_date = t.spend_date

{"headers": ["spend_date", "platform", "total_am", "total_u"], 。
"values": [
["2019-07-01", "desktop", 100, 1], 
["2019-07-02", "desktop", 100, 1], 
["2019-07-01", "mobile", 100, 1], 
["2019-07-02", "mobile", 100, 1], 
["2019-07-01", "both", 200, 1], 
["2019-07-02", "both", null, null]]}

• 对连接后的表，求和

# Write your MySQL query statement below
selectspend_date, platform,ifnull(sum(total_am),0) total_amount,ifnull(sum(total_u),0) total_users
from
(select p.spend_date, p.platform, t.total_am, t.total_ufrom(select distinct spend_date, "desktop" platform from Spendingunionselect distinct spend_date, "mobile" platform from Spendingunionselect distinct spend_date, "both" platform from Spending) pleft join(select spend_date, if(count(distinct platform)=1, platform, 'both') plat,sum(amount) total_am,count(distinct user_id) total_ufrom Spendinggroup by spend_date, user_id) ton p.platform = t.plat and p.spend_date = t.spend_date
) temp
group by spend_date, platform

---

我的CSDN博客地址 https://michael.blog.csdn.net/

长按或扫码关注我的公众号（Michael阿明），一起加油、一起学习进步！