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leetcode/problem/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 |
++----------------+---------+
+user_id is the primary key of this table.
+This table has the info of the users of an online shopping website where users can sell and buy items.
+</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     |
++---------------+---------+
+order_id is the primary key of this table.
+item_id is a foreign key to the Items table.
+buyer_id and seller_id are foreign keys to the Users table.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>Items</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| item_id       | int     |
+| item_brand    | varchar |
++---------------+---------+
+item_id is the primary key of this table.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write an SQL query to find for each user, the join date and the number of orders they made as a buyer in <code>2019</code>.</p>
+
+<p>Return the result table in <strong>any order</strong>.</p>
+
+<p>The query result format is in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+Users table:
++---------+------------+----------------+
+| 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 table:
++----------+------------+---------+----------+-----------+
+| 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 table:
++---------+------------+
+| item_id | item_brand |
++---------+------------+
+| 1       | Samsung    |
+| 2       | Lenovo     |
+| 3       | LG         |
+| 4       | HP         |
++---------+------------+
+<strong>Output:</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>