before delete, select trigger: table mutating error

Hi,
User is allowed to delete records, if they're not the last one in the group, otherwise will kick out an application error. I had the following codes, obviously I had table mutating error. How can I avoid it? I do want all next to last records in group deleted. Thanks.

create or replace TRIGGER trg_Patient_Visit_Detail_f2d  BEFORE DELETE ON patient_visit_detail_f  for each row
declare
 v_cnt number;
BEGIN
   select count(*) into v_cnt
   from patient_visit_detail_f
   where study_number=:old.study_number and

         investigator_number=:old.investigator_number and
         patient_number=:old.patient_number;
   IF v_cnt<2 then
     raise_application_error(-20002,'Delete not allowed, update

   end if;
END trg_Patient_Visit_Detail_f2d; Received on Mon Feb 25 2002 - 17:43:15 CET