merge into a 
using b
on (a.a=b.b)
when matched then update xxxxx
when not matched then insert (xxx) values(xxx);

------解决方案--------------------

declare
  vi number :=0;
  v_rowid varchar2(18);
begin
 begin
   select t.rowid  
   into v_rowid
   from tablexxx t
   where t.idxx = 'xxxid';
 exception
   when others then
     v_rowid := '';
 end;
 if v_rowid is null then
   insert into xxx......;
 else  
   update tablexxx t
   set  ......
   where t.rowid = v_rowid;
 end if;
exception
 when others then
   null;
end;