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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 Spendingunionselect distinct spend_date, "mobile" platform from Spendingunionselect 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 Spendinggroup 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_ufrom(    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) pleft join(    select spend_date,         if(count(distinct platform)=1, platform, 'both') plat,        sum(amount) total_am,        count(distinct user_id) total_u    from Spending    group by spend_date, user_id) ton 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 belowselect    spend_date, platform,    ifnull(sum(total_am),0) total_amount,    ifnull(sum(total_u),0) total_usersfrom(    select p.spend_date, p.platform, t.total_am, t.total_u    from    (        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    ) p    left join    (        select spend_date,             if(count(distinct platform)=1, platform, 'both') plat,            sum(amount) total_am,            count(distinct user_id) total_u        from Spending        group by spend_date, user_id    ) t    on p.platform = t.plat and p.spend_date = t.spend_date) tempgroup by spend_date, platform

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