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: N'ALFKI', N'FISSA', and N'PARIS':

SELECT
  C.CustomerID,
  COUNT(O.OrderID) AS OrderCount
FROM Customers C
LEFT JOIN Orders O
  ON O.CustomerID = C.CustomerID
WHERE C.CustomerID IN (N'ALFKI', N'FISSA', N'PARIS')
GROUP BY C.CustomerID;

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.OrderID) instead of COUNT(*). This ensures that we only count rows with non-NULL OrderID 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 CustomerID and a NULL value in the OrderID column.