Find Customer Referee

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Medium

Description

You have a Customer table where each customer may have been referred by another customer (stored as referee_id). Write a SQL query to find the names of customers who were NOT referred by the customer with id = 2. Return the result in any order.

Database Schema

Customer

Column Name	Type	Description
id	INT	Primary key
name	VARCHAR	Customer name
referee_id	INT	ID of customer who referred them (nullable)

Example

Customer

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

Output

name
Will
Jane
Bill
Zack

Explanation:

Alex and Mark were referred by customer 2. All others (including those with NULL referee_id) are returned.

Approach hint

Start with a simple approach, explain the trade-off, then move toward a cleaner or more scalable solution.

Common mistake

Skipping assumptions, edge cases, or trade-offs can make an otherwise good answer feel incomplete.