求帮忙修改个自定义函数啊!!!! 不会弄了!! 怎么if else啊

SQL code

  CREATE OR REPLACE FUNCTION "ETL_GET_111HS_CSTM_NO"(acode VARCHAR2)
  RETURN VARCHAR2 is
  pCM01CUSNO VARCHAR2(10);
BEGIN
  case
    when exists (select 1
            from sop_mir_cifm04
           where cm04cusno is not null
             and cm04acct = acode) then
      select cm04cusno
        into pCM01CUSNO
        from sop_mir_cifm04
       where cm04acct = acode
         and cm04cusno is not null;
    else
      SELECT CM01CUSNO into pCM01CUSNO FROM CIFM01 where CM01ACCNO = acode;
  end;
  RETURN pCM01CUSNO;
END;

怎么if else啊...

求大家帮助啊...

意思就是第一个表中 数据不为空的时候 拿第一个表中的数据项
如果第一个表的数据项为空
就return第二个表的数据项...

快帮助我哇!!!!!!!!!!!!!!!!!!!!!

------解决方案--------------------
CREATE OR REPLACE FUNCTION "ETL_GET_111HS_CSTM_NO"(acode VARCHAR2)
RETURN VARCHAR2 is
pCM01CUSNO VARCHAR2(10);
a1 varchar2(10);
a2 varchar2(10);
BEGIN
select cm04cusno into a1 from sop_mir_cifm04
where cm04cusno is not null
and cm04acct = acode ;

SELECT CM01CUSNO into a2 FROM CIFM01 where CM01ACCNO = acode;
return nvl(a1,a2);
END;