zOs/SQL/EXNULL

with u (u) as
( select 1 from sysibm.sysdummy1
  union all select 9 from sysibm.sysdummy1
  union all select cast( null as int) from sysibm.sysdummy1
)
select u
  , case when u  in (1,5) then 'in' else 'in else' end
  , case when u not in (1,5) then 'not in' else 'not in else' end
  , case when 1=0 or u in (1,5) then 'or in' else 'or in else' end
    from u
 -- where u not in (1,5)
;x;