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leetcode/problem/customer-placing-the-largest-number-of-orders.html

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+<p>Table: <code>Orders</code></p>
+
+<pre>
++-----------------+----------+
+| Column Name     | Type     |
++-----------------+----------+
+| order_number    | int      |
+| customer_number | int      |
++-----------------+----------+
+order_number is the primary key for this table.
+This table contains information about the order ID and the customer ID.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write an SQL query to find the <code>customer_number</code> for the customer who has placed <strong>the largest number of orders</strong>.</p>
+
+<p>The test cases are generated so that <strong>exactly one customer</strong> will have placed more orders than any other customer.</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> 
+Orders table:
++--------------+-----------------+
+| order_number | customer_number |
++--------------+-----------------+
+| 1            | 1               |
+| 2            | 2               |
+| 3            | 3               |
+| 4            | 3               |
++--------------+-----------------+
+<strong>Output:</strong> 
++-----------------+
+| customer_number |
++-----------------+
+| 3               |
++-----------------+
+<strong>Explanation:</strong> 
+The customer with number 3 has two orders, which is greater than either customer 1 or 2 because each of them only has one order. 
+So the result is customer_number 3.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Follow up:</strong> What if more than one customer has the largest number of orders, can you find all the <code>customer_number</code> in this case?</p>