unique-orders-and-customers-per-month.sql

/*

Unique Orders And Customers Per Month Problem


Description
LeetCode Problem 1565.

Table: Orders

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| order_id      | int     |
| order_date    | date    |
| customer_id   | int     |
| invoice       | int     |
+---------------+---------+
order_id is the primary key for this table.
This table contains information about the orders made by customer_id.
Write an SQL query to find the number of unique orders and the number of unique customers with invoices > $20 for each different month.

Return the result table sorted in any order.

The query result format is in the following example:

Orders
+----------+------------+-------------+------------+
| order_id | order_date | customer_id | invoice    |
+----------+------------+-------------+------------+
| 1        | 2020-09-15 | 1           | 30         |
| 2        | 2020-09-17 | 2           | 90         |
| 3        | 2020-10-06 | 3           | 20         |
| 4        | 2020-10-20 | 3           | 21         |
| 5        | 2020-11-10 | 1           | 10         |
| 6        | 2020-11-21 | 2           | 15         |
| 7        | 2020-12-01 | 4           | 55         |
| 8        | 2020-12-03 | 4           | 77         |
| 9        | 2021-01-07 | 3           | 31         |
| 10       | 2021-01-15 | 2           | 20         |
+----------+------------+-------------+------------+

Result table:
+---------+-------------+----------------+
| month   | order_count | customer_count |
+---------+-------------+----------------+
| 2020-09 | 2           | 2              |
| 2020-10 | 1           | 1              |
| 2020-12 | 2           | 1              |
| 2021-01 | 1           | 1              |
+---------+-------------+----------------+
In September 2020 we have two orders from 2 different customers with invoices > $20.
In October 2020 we have two orders from 1 customer, and only one of the two orders has invoice > $20.
In November 2020 we have two orders from 2 different customers but invoices < $20, so we don't include that month.
In December 2020 we have two orders from 1 customer both with invoices > $20.
In January 2021 we have two orders from 2 different customers, but only one of them with invoice > $20.

*/

# V0
select left(order_date, 7) as month, count(distinct order_id) as order_count, 
	count(distinct customer_id) as customer_count
from orders
where invoice > 20
group by month

# V1
# https://circlecoder.com/unique-orders-and-customers-per-month/
select left(order_date, 7) as month, count(distinct order_id) as order_count, 
	count(distinct customer_id) as customer_count
from orders
where invoice > 20
group by month

# V2
# Time:  O(n)
# Space: O(n)
SELECT LEFT(order_date, 7) month,
       COUNT(DISTINCT order_id) order_count,
       COUNT(DISTINCT customer_id) customer_count
FROM orders
WHERE invoice > 20
GROUP BY 1
ORDER BY NULL;