Has Alex purchased product with id 498?
WHEN ProductID = 498 AND FirstName = 'Alex' means that Alex purchased a product; WHEN ProductID = 498 AND FirstName NOT 'Alex' means Alex has not purchased a product;
SELECT IIF(T1.ProductID = 498, 'YES', 'NO') FROM Sales AS T1 INNER JOIN Customers AS T2 ON T1.CustomerID = T2.CustomerID WHERE T2.FirstName = 'Alex'