Case function in where clause

Question

i insert the below rows in a table cash_book.
PRJ_CODE PRJ_NAME
1203 SHIFA
1203 SHIFA
1203 SHIFA
1202 FATIMA
1202 FATIMA
1203 SHIFA
if i select 1202 code then return 1202 code rows, if i select 1203 then returns 1203 code rows, if i select other than both values then returns all rows. how can i define in one query with use (case or decode function) in where clause ??

Answer 1

perhaps something like this:
create table test
as
select 1200 + rownum prj_code
  from dual
connect by level <= 10;
select * from test;
  PRJ_CODE
      1201
      1202
      1203
      1204
      1205
      1206
      1207
      1208
      1209
      1210
var v_PRJ_CODE number
exec :v_PRJ_CODE := 1200;
select *
  from test
where PRJ_CODE = :v_PRJ_CODE
    or 1 = case when :v_PRJ_CODE in (1202, 1203) then 0 else 1 end;
  PRJ_CODE
      1201
      1202
      1203
      1204
      1205
      1206
      1207
      1208
      1209
      1210
exec :v_PRJ_CODE := 1203;
select *
  from test
where PRJ_CODE = :v_PRJ_CODE
    or 1 = case when :v_PRJ_CODE in (1202, 1203) then 0 else 1 end;
  PRJ_CODE
      1203Regards
Martin