Case Statement

i ahve one fucniton:
create function fntotalcountcustclas

( @campaign varchar(50), @startdate datetime, @enddate datetime)
RETURNS TABLE
AS
RETURN
( Select t.itemnmbr,t.custclas, t.custclasdescription, t.totalcustclas as totalcount

from
(
select
vi.itemnmbr, replace(vc.custclas,'','Unspecified') as custclas, vc.custclasdescription, count(vc.custclas) as totalcustclas
from vwcustnmbr vc
join vwitemnbmr vi
on vi.sopnumbe=vc.sopnumbe
Where vi.Campaign = @Campaign and (vc.docdate between @startdate and @enddate)

group by vi.itemnmbr,vc.custclas, vc.custclasdescription
) as t
)
when i m executing it:
select * from fntotalcountcustclas('copd','1/1/2008','4/11/2008')
order by totalcount desc

i m getting results like:
itemnmbr,custclas,custclasdescription,totalcount
------------------------------------------------
06-5841 STANDARD Standard(web) 31
06-5840 STANDARD Standard(web) 30
kr-014 STANDARD Standard(web) 72
06-5841 INDPATIENT Patient 12
06-5840 INDPATIENT Patient 9
06-5845 INDPATIENT Patient 6
06-5841 PROGRAM Program 6
06-5841 INST-HOSPITAL Hospital 11
...................

Basically, i ahve to use one condition to get corrrect output related to inputs:

like - i have to input @category varchar(50), @category_value varchar(50)
and if category = 'campaign' then category_value = ''
then output should be itemnmbr sum(totalcount) [whatever should be custclas or custclasdesscription]
itemnmbr sumcount
-----------------
06-5840 52
06-541 101
06-452 26
kr-045 252

and if categroy = 'item' then category_value = any itemnmbrs(06-5840,06-5845,06-5841 etc..)
then output should be
itemnmbr custclas custclasdescription totalcount
-----------------------------------------------------
06-5840 STANDARD Standard(web) 31
06-5840 INDPATIENT Patient 9
06-5840 PROGRAM Program 6
06-5840 INS-HOSPITAL Hospital 17

like that..

can anyone help me to write case statement.
thanks a lot!!

create function fntotalcountcustclas

( @campaign varchar(50), @startdate datetime, @enddate datetime,
@category varchar(50), @category_value varchar(50))
RETURNS TABLE
AS
RETURN
( Select t.itemnmbr,t.custclas, t.custclasdescription, t.totalcustclas as totalcount,
case when category

from
(
select
vi.itemnmbr, replace(vc.custclas,'','Unspecified') as custclas, vc.custclasdescription, count(vc.custclas) as totalcustclas
from vwcustnmbr vc
join vwitemnbmr vi
on vi.sopnumbe=vc.sopnumbe
Where vi.Campaign = @Campaign and (vc.docdate between @startdate and @enddate)

group by vi.itemnmbr,vc.custclas, vc.custclasdescription
) as t
)