You could use subqueries instead of joins, like this:
(SELECT TOP (1) C.ClassName_FD
FROM DBA.Class_TB AS C
WHERE C.ClassCode_FD = FI.strCcClassCode_FD
AND (C.Airline_FD = FI.strFiAirlineCode_FD OR C.Airline_FD = '')
ORDER BY C.Airline_FD DESC
) AS ClassName_FD,
FROM DBA.FoldItems_TB FI
Edit: You should also be able to use CROSS/OUTER APPLY to do the lookup if you prefer.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.