Your marketing team wants to know how many distinct customers have placed at least one order. Using the orders table, return a single value unique_customers — the count of distinct customer IDs.
orders
| column | type |
|---|---|
| id | INTEGER |
| customer_id | INTEGER |
| amount | NUMERIC |
| order_date | DATE |
| id | customer_id | amount | order_date |
|---|---|---|---|
| 1 | 101 | 49.99 | 2024-01-05 |
| 2 | 102 | 19.99 | 2024-01-10 |
| 3 | 101 | 99.99 | 2024-01-15 |
| 4 | 103 | 5.00 | 2024-01-20 |
| unique_customers |
|---|
| 3 |
Customer 101 appears twice but is counted only once. Three distinct customer IDs exist: 101, 102, and 103.