add leetcode problem-cn part1

算法题（国内版）/problem (Chinese)/市场分析 I [market-analysis-i].html

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+<p>Table: <code>Users</code></p>
+
+<pre>
++----------------+---------+
+| Column Name    | Type    |
++----------------+---------+
+| user_id        | int     |
+| join_date      | date    |
+| favorite_brand | varchar |
++----------------+---------+
+此表主键是 user_id。
+表中描述了购物网站的用户信息，用户可以在此网站上进行商品买卖。
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>Orders</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| order_id      | int     |
+| order_date    | date    |
+| item_id       | int     |
+| buyer_id      | int     |
+| seller_id     | int     |
++---------------+---------+
+此表主键是 order_id。
+外键是 item_id 和（buyer_id，seller_id）。
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>Items</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| item_id       | int     |
+| item_brand    | varchar |
++---------------+---------+
+此表主键是 item_id。
+</pre>
+
+<p>&nbsp;</p>
+
+<p>请写出一条SQL语句以查询每个用户的注册日期和在 <strong><code>2019</code> </strong>年作为买家的订单总数。</p>
+
+<p>以 <strong>任意顺序</strong> 返回结果表。</p>
+
+<p>查询结果格式如下。</p>
+
+<p>&nbsp;</p>
+
+<p><strong>示例 1:</strong></p>
+
+<pre>
+<strong>输入：</strong>
+Users 表:
++---------+------------+----------------+
+| user_id | join_date  | favorite_brand |
++---------+------------+----------------+
+| 1       | 2018-01-01 | Lenovo         |
+| 2       | 2018-02-09 | Samsung        |
+| 3       | 2018-01-19 | LG             |
+| 4       | 2018-05-21 | HP             |
++---------+------------+----------------+
+Orders 表:
++----------+------------+---------+----------+-----------+
+| order_id | order_date | item_id | buyer_id | seller_id |
++----------+------------+---------+----------+-----------+
+| 1        | 2019-08-01 | 4       | 1        | 2         |
+| 2        | 2018-08-02 | 2       | 1        | 3         |
+| 3        | 2019-08-03 | 3       | 2        | 3         |
+| 4        | 2018-08-04 | 1       | 4        | 2         |
+| 5        | 2018-08-04 | 1       | 3        | 4         |
+| 6        | 2019-08-05 | 2       | 2        | 4         |
++----------+------------+---------+----------+-----------+
+Items 表:
++---------+------------+
+| item_id | item_brand |
++---------+------------+
+| 1       | Samsung    |
+| 2       | Lenovo     |
+| 3       | LG         |
+| 4       | HP         |
++---------+------------+
+<strong>输出：</strong>
++-----------+------------+----------------+
+| buyer_id  | join_date  | orders_in_2019 |
++-----------+------------+----------------+
+| 1         | 2018-01-01 | 1              |
+| 2         | 2018-02-09 | 2              |
+| 3         | 2018-01-19 | 0              |
+| 4         | 2018-05-21 | 0              |
++-----------+------------+----------------+</pre>