Queries :: Return Word Unknown For Any Code That Doesn't Have A Match

I have lookup table I use to return names for various "Sales Class" codes.It all works good but if there is a code that isn't in the lookup table it leaves that field empty.I want it to return the word "Unknown" for any code that doesn't have a match.Here is the SQL:

Code:

SELECT [Data1].OrderNum, SalesClasses.[Name]
FROM [Data1]
LEFT JOIN SalesClasses ON [Data1].[Sales Class] = SalesClasses.[Code1];

Table examples:

Data1:
OrderNum - Sales Class
111 - class1
222 - class2
333 - classX

[code]...