586. Customer Placing the Largest Number of Orders

Description

Table: Orders

+-----------------+----------+
| Column Name     | Type     |
+-----------------+----------+
| order_number    | int      |
| customer_number | int      |
+-----------------+----------+
order_number is the primary key (column with unique values) for this table.
This table contains information about the order ID and the customer ID.

Write a solution to find the customer_number for the customer who has placed the largest number of orders.

The test cases are generated so that exactly one customer will have placed more orders than any other customer.

The result format is in the following example.

Example 1:

Input: 
Orders table:
+--------------+-----------------+
| order_number | customer_number |
+--------------+-----------------+
| 1            | 1               |
| 2            | 2               |
| 3            | 3               |
| 4            | 3               |
+--------------+-----------------+
Output: 
+-----------------+
| customer_number |
+-----------------+
| 3               |
+-----------------+
Explanation: 
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.

Follow up: What if more than one customer has the largest number of orders, can you find all the customer_number in this case?

Solutions

Solution 1: Group By + Sorting

We can use GROUP BY to group the data by customer_number, and then sort the groups in descending order by count(1). Finally, we can take the customer_number of the first record as the result.

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# Write your MySQL query statement below
SELECT
    customer_number
FROM orders
GROUP BY customer_number
ORDER BY COUNT(1) DESC
LIMIT 1;

Solution 2

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/* Write your T-SQL query statement below */
SELECT TOP 1
    customer_number
FROM
    orders
GROUP BY customer_number
ORDER BY COUNT(customer_number) DESC;

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