[LeetCode] 586. Customer Placing the Largest Number of Orders

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■ 문제

https://leetcode.com/problems/customer-placing-the-largest-number-of-orders/

Customer Placing the Largest Number of Orders - LeetCode

Table: Orders

+-----------------+----------+
| 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.

 

Write an SQL query 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 query 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?

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■ 풀이

  문제 요구사항  

주문(order_number)건수가 가장 많은 고객 번호(customer_number)를 출력해라

> 고객번호별로 주문건수를 count해주고 내림차순 정렬해서 주문건수가 가장 많은 고객번호를 출력하면 된다.

>> FROM절의 서브쿼리, LIMIT 활용

SELECT customer_number
 FROM (SELECT customer_number, count(*)
        FROM Orders
        GROUP BY customer_number
        ORDER BY 2 DESC) a
 LIMIT 1;