Write a SELECT query to retrieve each customer ID and the total number of orders from the orders table.

To get the total number of orders per customer you can use the COUNT aggregate function and GROUP BY. The query assumes the orders table has a column named customer_id that references the customer.

SELECT customer_id, COUNT(*) AS total_orders

FROM orders

GROUP BY customer_id;

Explanation: COUNT(*) counts all rows for each customer_id. GROUP BY groups the rows by customer_id so the count is calculated per customer. If you only want to count distinct orders you could use COUNT(DISTINCT order_id). The result will list each customer_id once with the number of orders they have placed. Complexity is O(n) where n is the number of rows in orders, as the database scans the table once and aggregates by key.