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Question

I have an existing TABLE postn_matrix which contains a list of employees and a count of their resp. positions in the organisation.

Whenever a position of a user is added or removed, the corresponding count is reflected in the table thro' this trigger (VIA UPDATE)

Now, if there is a new user, he will not have an entry in postn_matrix , so I have to insert a new record for him/her (VIA INSERT). This needs to be brought in from the BASE TABLE.

The update seems to be working fine but I am not able to bring in a new user into the table.

I've been trying to handle this case with a cursor. But it hasnt been of any help yet. I'm hoping some expert could show me the light.. :). any other suggestions besides using a cursor will be much appreciated

CREATE OR REPLACE TRIGGER TRIG1
BEFORE INSERT OR DELETE ON (BASETABLE)
FOR EACH ROW
DECLARE
  cursor c1 is
  select person_id 
  from postn_matrix;

  v_temp varchar2(15);
BEGIN
  IF INSERTING THEN
    open c1;    
    LOOP
      fetch c1 into v_temp;

      if v_temp!=:new.person_id THEN
        insert into POSTN_MATRIX (PERSON_ID)
        VALUES (:new.PERSON_ID);
      else
        UPDATE POSTN_MATRIX
        //this is working fine ;

      END IF;
    end loop;
    close c1;

END
/

Answer 1

Because of the loop (which is missing an exit clause - hopefully you've just lost that translating this into a question) you're going to attempt to insert a record into pstn_matrix for every record the cursor returns, whether there are any matching :new.person_id or not; and if there is match you'll also do the update . Which probably isn't what you want, and you might get a constraint violation among other things. You also aren't setting your counter field - if that is not nullable then that will error. But you haven't said what errors, if any, you are getting.

If you must do this through a trigger then you can either check if there is a row for the new person at all:

DECLARE
  v_temp postn_matrix.person_id%TYPE;
BEGIN
  IF INSERTING THEN
    select max(person_id) into v_temp
    from postn_matrix
    where person_id = :new.person_id;

    if v_temp is null then
      -- no record found, so insert one
      insert into postn_matrix (person_id, position_count)
      values (:new.person_id, 1);
    else
      -- record exists, so update
      update postn_matrix ...
    end if;
  ...

... or use merge .

But I don't like this model, and you're setting up the potential for data discrepancies with concurrent modifications to the base table. Trying to maintain a count like this is not necessarily as simple as it seems.

I'd usually prefer to make this a view, which will always be up to date and doesn't need the trigger complicating things:

create view postn_matrix as
  select person_id, count(*)
  from basetable
  group by person_id;

Of course, I may be misinterpreting or oversimplifying what your base table(s) does and what you need postn_matrix for. It seems a little trivial to have even as a view. If you have separate person and person_position tables, say, then you can add in an outer join to see people with no positions:

create view postn_matrix as
  select p.person_id, count(pp.position_id)
  from person p
  left join person_position pp on pp.person_id = p.person_id
  group by p.person_id;