Perfect! When writing reports in SQL, you have to remember that some objects may not exist. In this exercise we discuss a very common error in using COUNT() with LEFT JOIN. The next report we'd like to create should count orders for three different customer IDs: 'ALFKI', 'FISSA', and 'PARIS':

SELECT
  c.customer_id,
  COUNT(o.order_id) AS order_count
FROM customers c
LEFT JOIN orders o
  ON o.customer_id = c.customer_id
WHERE c.customer_id IN ('ALFKI', 'FISSA', 'PARIS')
GROUP BY c.customer_id;

Note the following:

1. We used LEFT JOIN to make sure we'll see all three customer IDs in the report. If we used a simple JOIN and any of the customers placed no orders, they would not be shown in the report.
2. We used COUNT(o.order_id) instead of COUNT(*). This ensures that we only count rows with non-NULL order_id column values. This is important if a customer hasn't ordered anything – in that case, COUNT(*) would return 1 instead of 0 because there would be one row with the given customer_id and a NULL value in the order_id column.