LeetCode 584: Find Customer Referee - Detailed Explanation & SQL Solution

In this article, we'll explore LeetCode Problem 584: Find Customer Referee, a relatively straightforward SQL query task. This problem challenges us to retrieve customer names based on specific conditions related to the referee_id field. We'll dive into the problem, break it down step by step, and discuss the solution with an optimized SQL query.

Problem Statement

We are given a table named Customer that contains the following columns:

id: an integer representing the customer ID.
name: a string representing the name of the customer.
referee_id: an integer referring to the ID of another customer who referred them (this can be null if no one referred them).

Our task is to write an SQL query to return the names of customers who were not referred by customer with ID = 2 or those who have a null value in the referee_id field.

Table Structure

Here's a sample structure of the Customer table:

id	name	referee_id
1	Will	NULL
2	Jane	NULL
3	Alex	2
4	Bill	NULL
5	Zack	1
6	Mark	2

Required Output

Based on the given problem, we need to filter out the customers who were referred by customer ID 2. If a customer was not referred by ID 2 or was not referred by anyone (i.e., referee_id is NULL), we include their name in the output.

For the above example, the output should look like this:

name
Will
Jane
Bill
Zack

SQL Query

To solve this problem, the SQL query will look like this:

SELECT name 
FROM Customer 
WHERE referee_id != 2 OR referee_id IS NULL;

Explanation of the Query

SELECT name: We are selecting the name column from the Customer table because that is the information we need to return.
FROM Customer: The data source is the Customer table.
WHERE referee_id != 2 OR referee_id IS NULL: This condition filters out all the customers who were referred by customer ID 2. It also includes customers who don't have a referee (i.e., their referee_id is NULL).

Breaking Down the Logic

The condition referee_id != 2 ensures that customers who were referred by customer ID 2 are excluded.
The condition referee_id IS NULL ensures that customers without any referee (i.e., NULL referee_id) are included in the result.
The OR operator ensures that either condition being true will include the customer in the final result set.

Optimized Query Considerations

In this particular problem, the solution is already efficient given the nature of the task and the simplicity of the dataset. If the Customer table were to grow significantly, indexing the referee_id column might help with query performance, particularly if it's used frequently in queries.

Sample Execution

Let’s run the SQL query step by step on the given table:

SELECT name 
FROM Customer 
WHERE referee_id != 2 OR referee_id IS NULL;

name
Will
Jane
Bill
Zack

This output shows the customers who either were not referred by customer ID 2 or were not referred by anyone (i.e., NULL referee_id).

Conclusion

LeetCode Problem 584: Find Customer Referee is an easy SQL problem that tests your understanding of NULL handling and conditional queries. By using the != operator along with the IS NULL condition, we can efficiently solve the problem and retrieve the desired results.

This solution demonstrates the power of basic SQL logic in filtering data based on multiple conditions, making it an important query to understand for any SQL practitioner preparing for coding interviews.

Key Takeaways

Use != to filter out specific values.
Understand how to use IS NULL to account for missing or null data.
The OR operator allows combining multiple conditions in SQL queries.

By mastering such simple but fundamental concepts, you'll be well-equipped to handle more advanced SQL queries as you progress in your learning.

This article covers the core concepts and the step-by-step explanation of solving LeetCode Problem 584. If you have any further questions or need clarification on the SQL solution, feel free to leave a comment or reach out!