Problem

Table: Orders

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| order_id      | int     |
| order_date    | date    |
| customer_id   | int     |
| invoice       | int     |
+---------------+---------+
order_id is the column with unique values for this table.
This table contains information about the orders made by customer_id.

Write a solution 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 result format is in the following example.

Examples

Example 1:

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Input: 
Orders table:
+----------+------------+-------------+------------+
| 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         |
+----------+------------+-------------+------------+
Output: 
+---------+-------------+----------------+
| month   | order_count | customer_count |
+---------+-------------+----------------+
| 2020-09 | 2           | 2              |
| 2020-10 | 1           | 1              |
| 2020-12 | 2           | 1              |
| 2021-01 | 1           | 1              |
+---------+-------------+----------------+
Explanation: 
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.

Solution

Method 1 – Group By and Filtering

Intuition

We need to count unique orders and unique customers with invoice > $20 for each month. We can filter, extract the month, and group by month.

Approach

  1. Filter orders with invoice > 20.
  2. Extract the month from order_date (e.g., ‘YYYY-MM’).
  3. Group by month, count distinct order_id and customer_id.

Code

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SELECT DATE_FORMAT(order_date, '%Y-%m') AS month,
       COUNT(DISTINCT order_id) AS order_count,
       COUNT(DISTINCT customer_id) AS customer_count
FROM Orders
WHERE invoice > 20
GROUP BY month;
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SELECT TO_CHAR(order_date, 'YYYY-MM') AS month,
       COUNT(DISTINCT order_id) AS order_count,
       COUNT(DISTINCT customer_id) AS customer_count
FROM Orders
WHERE invoice > 20
GROUP BY month;

Complexity

  • ⏰ Time complexity: O(n) — Each row is processed once.
  • 🧺 Space complexity: O(1) — Only a few variables for aggregation.