List all the products with lower than average cost.
cost refers to StandardCost; lower than average cost = StandardCost<(AVG(StandardCost));
SELECT DISTINCT T2.ProductID FROM ProductCostHistory AS T1 INNER JOIN Product AS T2 ON T1.ProductID = T2.ProductID WHERE T1.StandardCost < ( SELECT SUM(StandardCost) / COUNT(ProductID) FROM Product )