LeetCode 586. Customer Placing the Largest Number of Orders

Description

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

SQL Schema

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.

It is guaranteed that exactly one customer will have placed more orders than any other customer.

The query result format is in the following example:

Orders table:
+--------------+-----------------+
| order_number | customer_number |
+--------------+-----------------+
| 1            | 1               |
| 2            | 2               |
| 3            | 3               |
| 4            | 3               |
+--------------+-----------------+

Result table:
+-----------------+
| customer_number |
+-----------------+
| 3               |
+-----------------+
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 have the largest number of orders, can you find all the customer_number in this case?

Explanation

group by, then order by desc and limit 1.

SQL Solution

# Write your MySQL query statement below
select customer_number from Orders group by customer_number order by count(order_number) desc LIMIT 1