Customer Placing the Largest Number of Orders

Customer Placing the Largest Number of Orders

What is this problem about?

The Customer Placing the Largest Number of Orders interview question is a fundamental data analysis task. Given an Orders table containing order_number and customer_number, you need to identify the customer who has placed the highest volume of orders. This Customer Placing the Largest Number of Orders coding problem is a test of basic ranking and aggregation.

Why is this asked in interviews?

Companies like Microsoft and X (Twitter) use this to check for basic SQL proficiency. It evaluates if a candidate can count occurrences, group data correctly, and sort results to find a global maximum. It’s a foundational query that every developer working with relational databases should be able to write quickly and accurately.

Algorithmic pattern used

This follows the standard Database interview pattern of "Count-Group-Sort-Limit."

1. Group: Group the rows by customer_number.
2. Count: Use COUNT(*) to find the total orders for each group.
3. Sort: Order the results by the count in descending order.
4. Limit: Use LIMIT 1 (or TOP 1 depending on the SQL dialect) to retrieve only the top record.

Example explanation

Suppose the table has these entries:

• Order 1: Customer 101
• Order 2: Customer 102
• Order 3: Customer 101
• Order 4: Customer 103 Counting these gives: Customer 101 (2 orders), Customer 102 (1 order), Customer 103 (1 order). Sorting by count descending puts 101 at the top. The output is 101.

Common mistakes candidates make

• Forgetting the Group By: Attempting to count without grouping, which results in a single count of all orders rather than per customer.
• Handling Ties: Many candidates provide a solution that only returns one customer even if there is a tie. While some versions of this problem only ask for one, a more robust solution might use RANK() or a subquery to find all top customers.
• Confusing Count and Sum: Trying to SUM the order_number instead of counting the number of rows.

Interview preparation tip

Always ask if you should handle ties. If multiple customers have the same maximum number of orders, knowing whether to return all of them or just one helps you decide between a simple LIMIT or a more advanced window function.