add 02

Author: trinhminhtriet

.vscode/settings.json

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-    "editor.formatOnPaste": true
+    "editor.formatOnSave": true
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README.md

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 # Leetcode | Top SQL 50
 
-01. [Recyclable and Low Fat Products](solutions/01.1757.recyclable-and-low-fat-products/README.md)
+1.  [Recyclable and Low Fat Products](solutions/01.1757.recyclable-and-low-fat-products/README.md)
+2.  [Find Customer Referee](solutions/02.0584.find-customer-referee/README.md)

solutions/02.0584.find-customer-referee/README.md

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+---
+comments: true
+difficulty: Easy
+tags:
+  - Database
+---
+
+<!-- problem:start -->
+
+# [584. Find Customer Referee](https://leetcode.com/problems/find-customer-referee)
+
+## Description
+
+<!-- description:start -->
+
+<p>Table: <code>Customer</code></p>
+
+<pre>
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| id          | int     |
+| name        | varchar |
+| referee_id  | int     |
++-------------+---------+
+In SQL, id is the primary key column for this table.
+Each row of this table indicates the id of a customer, their name, and the id of the customer who referred them.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Find the names of the customer that are <strong>not referred by</strong> the customer with <code>id = 2</code>.</p>
+
+<p>Return the result table in <strong>any order</strong>.</p>
+
+<p>The result format is in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+Customer table:
++----+------+------------+
+| id | name | referee_id |
++----+------+------------+
+| 1  | Will | null       |
+| 2  | Jane | null       |
+| 3  | Alex | 2          |
+| 4  | Bill | null       |
+| 5  | Zack | 1          |
+| 6  | Mark | 2          |
++----+------+------------+
+<strong>Output:</strong> 
++------+
+| name |
++------+
+| Will |
+| Jane |
+| Bill |
+| Zack |
++------+
+</pre>
+
+<!-- description:end -->
+
+## Solutions
+
+<!-- solution:start -->
+
+### Solution 1: Conditional Filtering
+
+We can directly filter out the customer names whose `referee_id` is not `2`. Note that the customers whose `referee_id` is `NULL` should also be filtered out.
+
+<!-- tabs:start -->
+
+#### MySQL
+
+```sql
+# Write your MySQL query statement below
+SELECT name
+FROM Customer
+WHERE IFNULL(referee_id, 0) != 2;
+```
+
+<!-- tabs:end -->
+
+<!-- solution:end -->
+
+<!-- problem:end -->