Hopper Company Queries I
<p>Table: <code>Drivers</code></p>

<pre>
+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| driver_id   | int     |
| join_date   | date    |
+-------------+---------+
driver_id is the primary key (column with unique values) for this table.
Each row of this table contains the driver&#39;s ID and the date they joined the Hopper company.
</pre>

<p>&nbsp;</p>

<p>Table: <code>Rides</code></p>

<pre>
+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| ride_id      | int     |
| user_id      | int     |
| requested_at | date    |
+--------------+---------+
ride_id is the primary key (column with unique values) for this table.
Each row of this table contains the ID of a ride, the user&#39;s ID that requested it, and the day they requested it.
There may be some ride requests in this table that were not accepted.
</pre>

<p>&nbsp;</p>

<p>Table: <code>AcceptedRides</code></p>

<pre>
+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| ride_id       | int     |
| driver_id     | int     |
| ride_distance | int     |
| ride_duration | int     |
+---------------+---------+
ride_id is the primary key (column with unique values) for this table.
Each row of this table contains some information about an accepted ride.
It is guaranteed that each accepted ride exists in the Rides table.
</pre>

<p>&nbsp;</p>

<p>Write a solution to report the following statistics for each month of <strong>2020</strong>:</p>

<ul>
	<li>The number of drivers currently with the Hopper company by the end of the month (<code>active_drivers</code>).</li>
	<li>The number of accepted rides in that month (<code>accepted_rides</code>).</li>
</ul>

<p>Return the result table ordered by <code>month</code> in ascending order, where <code>month</code> is the month&#39;s number (January is <code>1</code>, February is <code>2</code>, etc.).</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> 
Drivers table:
+-----------+------------+
| driver_id | join_date  |
+-----------+------------+
| 10        | 2019-12-10 |
| 8         | 2020-1-13  |
| 5         | 2020-2-16  |
| 7         | 2020-3-8   |
| 4         | 2020-5-17  |
| 1         | 2020-10-24 |
| 6         | 2021-1-5   |
+-----------+------------+
Rides table:
+---------+---------+--------------+
| ride_id | user_id | requested_at |
+---------+---------+--------------+
| 6       | 75      | 2019-12-9    |
| 1       | 54      | 2020-2-9     |
| 10      | 63      | 2020-3-4     |
| 19      | 39      | 2020-4-6     |
| 3       | 41      | 2020-6-3     |
| 13      | 52      | 2020-6-22    |
| 7       | 69      | 2020-7-16    |
| 17      | 70      | 2020-8-25    |
| 20      | 81      | 2020-11-2    |
| 5       | 57      | 2020-11-9    |
| 2       | 42      | 2020-12-9    |
| 11      | 68      | 2021-1-11    |
| 15      | 32      | 2021-1-17    |
| 12      | 11      | 2021-1-19    |
| 14      | 18      | 2021-1-27    |
+---------+---------+--------------+
AcceptedRides table:
+---------+-----------+---------------+---------------+
| ride_id | driver_id | ride_distance | ride_duration |
+---------+-----------+---------------+---------------+
| 10      | 10        | 63            | 38            |
| 13      | 10        | 73            | 96            |
| 7       | 8         | 100           | 28            |
| 17      | 7         | 119           | 68            |
| 20      | 1         | 121           | 92            |
| 5       | 7         | 42            | 101           |
| 2       | 4         | 6             | 38            |
| 11      | 8         | 37            | 43            |
| 15      | 8         | 108           | 82            |
| 12      | 8         | 38            | 34            |
| 14      | 1         | 90            | 74            |
+---------+-----------+---------------+---------------+
<strong>Output:</strong> 
+-------+----------------+----------------+
| month | active_drivers | accepted_rides |
+-------+----------------+----------------+
| 1     | 2              | 0              |
| 2     | 3              | 0              |
| 3     | 4              | 1              |
| 4     | 4              | 0              |
| 5     | 5              | 0              |
| 6     | 5              | 1              |
| 7     | 5              | 1              |
| 8     | 5              | 1              |
| 9     | 5              | 0              |
| 10    | 6              | 0              |
| 11    | 6              | 2              |
| 12    | 6              | 1              |
+-------+----------------+----------------+
<strong>Explanation:</strong> 
By the end of January --&gt; two active drivers (10, 8) and no accepted rides.
By the end of February --&gt; three active drivers (10, 8, 5) and no accepted rides.
By the end of March --&gt; four active drivers (10, 8, 5, 7) and one accepted ride (10).
By the end of April --&gt; four active drivers (10, 8, 5, 7) and no accepted rides.
By the end of May --&gt; five active drivers (10, 8, 5, 7, 4) and no accepted rides.
By the end of June --&gt; five active drivers (10, 8, 5, 7, 4) and one accepted ride (13).
By the end of July --&gt; five active drivers (10, 8, 5, 7, 4) and one accepted ride (7).
By the end of August --&gt; five active drivers (10, 8, 5, 7, 4) and one accepted ride (17).
By the end of September --&gt; five active drivers (10, 8, 5, 7, 4) and no accepted rides.
By the end of October --&gt; six active drivers (10, 8, 5, 7, 4, 1) and no accepted rides.
By the end of November --&gt; six active drivers (10, 8, 5, 7, 4, 1) and two accepted rides (20, 5).
By the end of December --&gt; six active drivers (10, 8, 5, 7, 4, 1) and one accepted ride (2).
</pre>
Database
# Write your MySQL query statement below
WITH
    recursive Months AS (
        SELECT
            1 AS month
        UNION ALL
        SELECT
            month + 1
        FROM Months
        WHERE month < 12
    ),
    Ride AS (
        SELECT MONTH(requested_at) AS month, COUNT(1) AS cnt
        FROM
            Rides AS r
            JOIN AcceptedRides AS a
                ON r.ride_id = a.ride_id AND YEAR(requested_at) = 2020
        GROUP BY month
    )
SELECT
    m.month,
    COUNT(driver_id) AS active_drivers,
    IFNULL(r.cnt, 0) AS accepted_rides
FROM
    Months AS m
    LEFT JOIN Drivers AS d
        ON (m.month >= MONTH(d.join_date) AND YEAR(d.join_date) = 2020)
        OR YEAR(d.join_date) < 2020
    LEFT JOIN Ride AS r ON m.month = r.month
GROUP BY month;
