Count rows returned in a subquery in pl/sql block

Question

Hi,
I'm working on a trigger in Oracle 11 to check that a student has met prerequisites before enrolling in a class. I need to determine how many rows came back on the query in order to determine if I should raise an exception or not. The following is raising the exception if the prereqs are complete or not. I've haven't included the code in the where clause because I've tested is seperately and get good results. I know this is limiited info, but is there some obvious mistake I've made here?
CREATE OR REPLACE TRIGGER prereq
BEFORE INSERT ON enrollment
FOR EACH ROW
DECLARE
  howmany  number;
BEGIN
  select count(*)
  into howmany
  from dual
  where exists ((insert code here that returns 0 rows if prerequisites are not met);
  if howmany = 0 then
    raise_application_error(-20000, 'The prerequisites for this course are not complete');
  end if;
end;
/

Answer 1

As you are selecting from dual you might hit no_data_found if the exist caluse doesnot return any row.
Try this,
CREATE OR REPLACE TRIGGER prereq
   BEFORE INSERT
   ON enrollment
   FOR EACH ROW
DECLARE
   howmany        NUMBER;
BEGIN
   SELECT COUNT (*) INTO howmany FROM DUAL;
               WHERE EXISTS ((INSERT code here that returns 0 ROWS IF prerequisites are NOT met);
EXCEPTION
   WHEN NO_DATA_FOUND
   THEN
      raise_application_error (-20000
                              ,'The prerequisites for this course are not complete');
END;You dont need count(*) from dual you can simply use
SELECT 1 INTO howmany FROM DUAL;
               WHERE EXISTS ...G.