How to use join in a case statement

Question

Code below is not working. I want to pull the BatchHorizon of the corresponding PACKAGE. I cannot directly use "then BatchHorizon " since the batchhorizon is inside the subquery.

Case When PACKAGE IN (SELECT DISTINCT PACKAGE
                      FROM BATCHINGWINDOW AS A
                     ) 
     Then (SELECT  BatchHorizon
           FROM BATCHINGWINDOW AS B 
           WHERE B.PACKAGE = A.PACKAGE
         )
     else 3
 end

Answer 1

select 

Case When O.PACKAGE IN (SELECT PACKAGE
                      FROM BATCHINGWINDOW  
                     ) 
     Then (SELECT  DISTINCT BatchHorizon
           FROM BATCHINGWINDOW AS B 
           WHERE B.PACKAGE = O.PACKAGE
         )
     else 3
 end pack
 from table O