List all the numbers ordered by 'Rochester Ltd' in 2018.
Rochester Ltd is the name of the customer; all the numbers ordered refer to OrderNumber; 2018 refers to SUBSTR(OrderDate, -2) = '18';
SELECT DISTINCT T FROM ( SELECT CASE  WHEN T1.OrderDate LIKE '%/%/18' AND T2.`Customer Names` = 'Rochester Ltd' THEN T1.OrderNumber ELSE NULL END AS T FROM `Sales Orders` T1 INNER JOIN Customers T2 ON T2.CustomerID = T1._CustomerID ) WHERE T IS NOT NULL