Added README.md file for Replace Employee ID With The Unique Identifier

Author: Pratik-Khose

1509-replace-employee-id-with-the-unique-identifier/README.md

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+<h2><a href="https://leetcode.com/problems/replace-employee-id-with-the-unique-identifier">Replace Employee ID With The Unique Identifier</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Table: <code>Employees</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| id            | int     |
+| name          | varchar |
++---------------+---------+
+id is the primary key (column with unique values) for this table.
+Each row of this table contains the id and the name of an employee in a company.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>EmployeeUNI</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| id            | int     |
+| unique_id     | int     |
++---------------+---------+
+(id, unique_id) is the primary key (combination of columns with unique values) for this table.
+Each row of this table contains the id and the corresponding unique id of an employee in the company.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write a solution to show the <strong>unique ID </strong>of each user, If a user does not have a unique ID replace just show <code>null</code>.</p>
+
+<p>Return the result table in <strong>any</strong> order.</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> 
+Employees table:
++----+----------+
+| id | name     |
++----+----------+
+| 1  | Alice    |
+| 7  | Bob      |
+| 11 | Meir     |
+| 90 | Winston  |
+| 3  | Jonathan |
++----+----------+
+EmployeeUNI table:
++----+-----------+
+| id | unique_id |
++----+-----------+
+| 3  | 1         |
+| 11 | 2         |
+| 90 | 3         |
++----+-----------+
+<strong>Output:</strong> 
++-----------+----------+
+| unique_id | name     |
++-----------+----------+
+| null      | Alice    |
+| null      | Bob      |
+| 2         | Meir     |
+| 3         | Winston  |
+| 1         | Jonathan |
++-----------+----------+
+<strong>Explanation:</strong> 
+Alice and Bob do not have a unique ID, We will show null instead.
+The unique ID of Meir is 2.
+The unique ID of Winston is 3.
+The unique ID of Jonathan is 1.
+</pre>