Customer Placing the Largest Number of Orders

LeetCode 586 | Difficulty: Easy

Easy

Problem 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?

Topics: Database

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Approach

Direct Approach

This problem can typically be solved with straightforward iteration or simple data structure usage. Focus on correctness first, then optimize.

When to use

Basic problems that test fundamental programming skills.

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Solutions

Solution 1: MySQL (Best: 1191 ms)

Metric	Value
Runtime	1191 ms
Memory	N/A
Date	2018-05-08

Solution

# Write your MySQL query statement below
SELECT customer_number
        
    FROM orders O
    GROUP BY customer_number
    ORDER BY count(*) DESC
    limit 1

📜 1 more MySQL submission(s)

Submission (2018-05-08) — 1235 ms, N/A

# Write your MySQL query statement below
select customer_number from (select customer_number, count(*) as cnt from orders O group by customer_number order by count(*) desc) AS A limit 1

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Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: MySQL uses a different expression to get the first records other than MSSQL's TOP expression.