How do you count the number of orders per customer?

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

Answer: (B)

To count how many orders each customer placed, you group the data by the customer identifier and count the rows in each group. GROUP BY customer_id creates one result row for every customer, and COUNT(*) tallies every row in that group, giving the number of orders for that customer. This yields a per-customer breakdown with the correct count.

Using a sum on an order_count column would require such a column to already hold the per-row counts, which isn’t typically the case in a raw orders table. Counting a specific column (COUNT(column_name)) counts only non-null values in that column, which can undercount if there are NULLs. And counting without a GROUP BY would return a single total for all customers, losing the per-customer detail.

So grouping by the customer and using COUNT(*) is the correct approach to obtain the number of orders per customer.