what are the actual delivery dates of orders with quantity 1?
SELECT T1.Actual_Delivery_Date FROM Customer_Orders AS T1 JOIN ORDER_ITEMS AS T2 ON T1.Order_ID   =   T2.Order_ID WHERE T2.Order_Quantity   =   1
actual delivery dates of orders refers to T1.Actual_Delivery_Date; orders with quantity 1 refers to T2.Order_Quantity = 1; JOIN is used to combine rows from two or more tables based on a related column between them, in this case, Order_ID.