Can't use temporary column in where clause?

Question

select  cast(de.ApprovalOrder AS VARCHAR(32)) 
            + cast(de.EntityCode AS VARCHAR(32)) 
            + isnull(cast(de.DelegationCode AS VARCHAR(32)), '') as 'RowID' ,
            *
from    workflow.delegation_engine de
where   RowID <> NULL

When I try to execute the following I receive the error:

Msg 207, Level 16, State 1, Line 13 Invalid column name 'RowID'.

Just wondering how I can reference this temporary column? I searched for previous postings which suggested using 'having' for this however that doesn't appear to work either.