Multi Value Parameter in a Query.

Can someone guide me thru on how to make this happen, I have a report to generate which will let a user select Customer,SalesRep,Location & Product. I need to write a query which will let the user the parameter value, It’s basically a multifunctional query  that generates whatever is selected on the pic list.  If you select “N” for anything it then allows you to select other criteria.  For instance…I may want to see a monthly selection of shipments of Special Select to all customers, or a selection of customers.  Or….a selection of all fish meal products to a group of 10 customers….and so on

select distinct T1. C01_CMM_S , 
T1. c01_GROSS_DATE_T_AS_A_DATE , 
T2. C05_CMM_DESC_S as Prodname , 

case  when {fn MONTH(T1. c01_GROSS_DATE_T_AS_A_DATE )} = 1 then case  when T1. C01_CMM_S  in ('A5', 'B2', 'B5', 'B6', 'D2') 
then T1. C07_CUS_NET_UNTS_D  / 2204.6 when T1. C01_CMM_S  in ('G1', 'G3', 'L1', 'N1', 'O1') then T1. C07_CUS_NET_UNTS_D  * 1.1023 
when T1. C01_CMM_S  = 'O2' then T1. C07_CUS_NET_UNTS_D  / 2000 else T1. C07_CUS_NET_UNTS_D  end  else 0 end January, 

{fn YEAR(T1. c01_GROSS_DATE_T_AS_A_DATE )} Month, 

case  when {fn MONTH(T1. c01_GROSS_DATE_T_AS_A_DATE )} = 2 then case  when T1. C01_CMM_S  in ('A5', 'B2', 'B5', 'B6', 'D2') 
then T1. C07_CUS_NET_UNTS_D  / 2204.6 when T1. C01_CMM_S  in ('G1', 'G3', 'L1', 'N1', 'O1') then T1. C07_CUS_NET_UNTS_D  * 1.1023 
when T1. C01_CMM_S  = 'O2' then T1. C07_CUS_NET_UNTS_D  / 2000 else T1. C07_CUS_NET_UNTS_D  end  else 0 end Febuary , 

case  when {fn MONTH(T1. c01_GROSS_DATE_T_AS_A_DATE )} = 3 then case  when T1. C01_CMM_S  in ('A5', 'B2', 'B5', 'B6', 'D2') 
then T1. C07_CUS_NET_UNTS_D  / 2204.6 when T1. C01_CMM_S  in ('G1', 'G3', 'L1', 'N1', 'O1') then T1. C07_CUS_NET_UNTS_D  * 1.1023 
when T1. C01_CMM_S  = 'O2' then T1. C07_CUS_NET_UNTS_D  / 2000 else T1. C07_CUS_NET_UNTS_D  end  else 0 end March, 



T3. Primary_Sales 

from   C05_COMMODITY_CODE  T2,   trns_Agris_shipment  
T1 LEFT OUTER JOIN   pres_ShipToCust_specie_Salesrep  T3 on T1. C01_CMM_S  = T3. C01_CMM_S  and T1. c01_shp_to_fr_id_s  = T3. N01_NAM_ID_S 
where T1. C01_CMM_S  = T2. C05_COMM_CD_S  
and (T1. C01_CMM_S  in (@Product) or @AllProduct = 'Y') 
and {fn YEAR(T1. c01_GROSS_DATE_T_AS_A_DATE )} = @Year and T1. c01_tkt_type_s  = 'R' and T1. c01_weight_base_s  in ('F') 
and (T1. c01_shp_to_fr_id_s  in (@Ship2Cust) or @AllShipto = 'Y') and (T3. Primary_Sales  in (@Salesrep) or @AllSalesRep= 'Y') 
and (T1. C01_LOC_S  in (@Location) or @AllLocation = 'Y')
 order by 7 asc , 1 asc , 8 asc , 9 asc , 10 asc , 12 asc

I have 8 Parameters in this query If the value of Parameter is 'Y' to select all Prods, location.customers,salesreps it selects all and If its entered as 'N' then a user should have ability to Multi select the value..

Does that make sense? Please Help !!!

FM