Added README.md file for Confirmation Rate

Author: sayandip20

2087-confirmation-rate/README.md

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+<h2><a href="https://leetcode.com/problems/confirmation-rate">Confirmation Rate</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><p>Table: <code>Signups</code></p>
+
+<pre>
++----------------+----------+
+| Column Name    | Type     |
++----------------+----------+
+| user_id        | int      |
+| time_stamp     | datetime |
++----------------+----------+
+user_id is the column of unique values for this table.
+Each row contains information about the signup time for the user with ID user_id.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>Confirmations</code></p>
+
+<pre>
++----------------+----------+
+| Column Name    | Type     |
++----------------+----------+
+| user_id        | int      |
+| time_stamp     | datetime |
+| action         | ENUM     |
++----------------+----------+
+(user_id, time_stamp) is the primary key (combination of columns with unique values) for this table.
+user_id is a foreign key (reference column) to the Signups table.
+action is an ENUM (category) of the type (&#39;confirmed&#39;, &#39;timeout&#39;)
+Each row of this table indicates that the user with ID user_id requested a confirmation message at time_stamp and that confirmation message was either confirmed (&#39;confirmed&#39;) or expired without confirming (&#39;timeout&#39;).
+</pre>
+
+<p>&nbsp;</p>
+
+<p>The <strong>confirmation rate</strong> of a user is the number of <code>&#39;confirmed&#39;</code> messages divided by the total number of requested confirmation messages. The confirmation rate of a user that did not request any confirmation messages is <code>0</code>. Round the confirmation rate to <strong>two decimal</strong> places.</p>
+
+<p>Write a solution to find the <strong>confirmation rate</strong> of each user.</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> 
+Signups table:
++---------+---------------------+
+| user_id | time_stamp          |
++---------+---------------------+
+| 3       | 2020-03-21 10:16:13 |
+| 7       | 2020-01-04 13:57:59 |
+| 2       | 2020-07-29 23:09:44 |
+| 6       | 2020-12-09 10:39:37 |
++---------+---------------------+
+Confirmations table:
++---------+---------------------+-----------+
+| user_id | time_stamp          | action    |
++---------+---------------------+-----------+
+| 3       | 2021-01-06 03:30:46 | timeout   |
+| 3       | 2021-07-14 14:00:00 | timeout   |
+| 7       | 2021-06-12 11:57:29 | confirmed |
+| 7       | 2021-06-13 12:58:28 | confirmed |
+| 7       | 2021-06-14 13:59:27 | confirmed |
+| 2       | 2021-01-22 00:00:00 | confirmed |
+| 2       | 2021-02-28 23:59:59 | timeout   |
++---------+---------------------+-----------+
+<strong>Output:</strong> 
++---------+-------------------+
+| user_id | confirmation_rate |
++---------+-------------------+
+| 6       | 0.00              |
+| 3       | 0.00              |
+| 7       | 1.00              |
+| 2       | 0.50              |
++---------+-------------------+
+<strong>Explanation:</strong> 
+User 6 did not request any confirmation messages. The confirmation rate is 0.
+User 3 made 2 requests and both timed out. The confirmation rate is 0.
+User 7 made 3 requests and all were confirmed. The confirmation rate is 1.
+User 2 made 2 requests where one was confirmed and the other timed out. The confirmation rate is 1 / 2 = 0.5.
+</pre>