Which statement would count the number of orders per customer using a GROUP BY clause?

• A. SELECT customer_id, COUNT(*) AS order_count FROM orders GROUP BY customer_id;
• B. SELECT customer_id, SUM(*) FROM orders GROUP BY customer_id;
• C. SELECT customer_id, COUNT(*) FROM orders;
• D. SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_id HAVING COUNT(*) > 0;

Answer: (A)

Grouping data by customer_id and counting rows in each group is how you get the number of orders per customer. In this approach, you select the customer_id and use an aggregate function COUNT() to count the rows in each group, then you apply GROUP BY customer_id to create a separate group for each customer. The alias order_count makes the result clear. This yields one row per customer with their total number of orders. If you omitted GROUP BY, you would get a single total for all orders, not per customer. The option that uses SUM() is invalid because SUM expects a numeric expression, not a set of rows. A version with HAVING COUNT(*) > 0 would work, but it's redundant since every group formed by GROUP BY has at least one row.