Update readme.md

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LeetCode SQL 50 Solution/584. Find Customer Referee/readme.md

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+Below is a structured README.md snippet for **LeetCode 584: Find Customer Referee**:
+
+---
+
+# **584. Find Customer Referee**
+
+## **Problem Statement**
+You are given a table `Customer` that stores customer details along with the ID of the customer who referred them.
+
+### **Customer Table**
+```
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| id          | int     |
+| name        | varchar |
+| referee_id  | int     |
++-------------+---------+
+```
+- `id` is the **primary key**.
+- Each row represents a customer with their `id`, `name`, and `referee_id`.
+- `referee_id` indicates the customer who referred them. It can be **NULL** if no one referred the customer.
+
+### **Task:**
+Find the names of the customers who are **not referred** by the customer with `id = 2`.
+
+---
+
+## **Example 1:**
+
+### **Input:**
+#### **Customer Table**
+```
++----+------+------------+
+| 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** (id = 3) and **Mark** (id = 6) are referred by the customer with `id = 2` and are excluded.
+- The remaining customers (**Will**, **Jane**, **Bill**, **Zack**) are not referred by the customer with `id = 2`.
+
+---
+
+## **Solution Approaches**
+
+### **SQL Solution (Using WHERE Clause)**
+```sql
+SELECT name
+FROM Customer
+WHERE referee_id != 2 OR referee_id IS NULL;
+```
+**Explanation:**
+- The query selects customer names where `referee_id` is either not equal to `2` or is `NULL`.
+- This effectively filters out customers referred by the customer with `id = 2`.
+
+---
+
+### **Pandas Solution**
+```python
+import pandas as pd
+
+def find_customer_referee(customer: pd.DataFrame) -> pd.DataFrame:
+    # Filter rows where referee_id is not equal to 2 or is null
+    result = customer[(customer['referee_id'] != 2) | (customer['referee_id'].isnull())][['name']]
+    return result
+```
+**Explanation:**
+- The Pandas solution filters the DataFrame for rows where `referee_id` is not 2 or is `NaN`.
+- It then returns the `name` column containing the desired customer names.
+
+---
+
+## **File Structure**
+```
+LeetCode584/
+├── problem_statement.md       # Contains the problem description and constraints.
+├── sql_solution.sql           # Contains the SQL solution.
+├── pandas_solution.py         # Contains the Pandas solution.
+├── README.md                  # Overview of the problem and solutions.
+```
+
+---
+
+## **Useful Links**
+- [LeetCode Problem 584](https://leetcode.com/problems/find-customer-referee/)
+- [SQL WHERE Clause Documentation](https://www.w3schools.com/sql/sql_where.asp)
+- [Pandas Filtering DataFrames](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.html)
+
+---
+
+This structured format provides a clear problem overview, multiple solution approaches, file organization, and useful references to help contributors understand and work on the problem. Happy coding! 🚀