Is this possible with query in post_query trigger?

Hi,
I have the following statements in my post-query trigger.
The query 
returns correctly when the ONLY the :BUD_QRY.A_BUD_YR is not
null. 
Nothing returned when the rest of other fields were not
NULL. Can I have 
a query like it?

Thank you.
Regards
Alan


DECLARE
    blk_id BLOCK;
    final_qry varchar2(4000);
    center_qry varchar2(50);
    costitem_qry varchar2(50);
    allo_qry varchar2(50);
    req_by_qry varchar2(50);
begin
    blk_id :=Find_block('VW_REQLST08');

    :tot.subtotal:=NULL;

 
    if :BUD_QRY.CENTER_ID is  not null
        then 
        center_qry:=' and CENTER_ID= :BUD_QRY.CENTER_ID';
    end if;
 
 if :BUD_QRY.costitem is not null
        then
        costitem_qry:=' and cost_item_id =
:BUD_QRY.costitem';
 end if;
 
 if :BUD_QRY.allo is not null
        then
        allo_qry:=' and allo_id = :BUD_QRY.allo';
    end if;
     
  if :BUD_QRY.req_by is not null
        then
        req_by_qry:=' and req_by = :BUD_QRY.req_by';
    end if;
  
   /* final_qry := 'BUD_YEAR= :BUD_QRY.A_BUD_YR ' || 
center_qry||costitem_qry||allo_qry || req_by_qry;*/

    select sum(subtotal) into :tot.subtotal from vw_reqlst08
where 
BUD_YEAR= :BUD_QRY.A_BUD_YR  ||
center_qry||costitem_qry||allo_qry || 
req_by_qry;

end;





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-- 
Author: Alan Kong
  INET: kkkongee.cuhk.edu.hk

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