Oracle: Using subquery in a trigger

Question

How can I work around the Oracle's limitation of not allowing subqueries in triggers.

Here's an example trigger I'm trying to create, but am unable to because I can't use a subquery.

CREATE OR REPLACE TRIGGER trigger_w_subquery
AFTER UPDATE OR INSERT ON project_archiving
FOR EACH ROW WHEN (old.archiving_status <> new.archiving_status
  AND new.archiving_status = 1
  AND (SELECT offer FROM projects WHERE projnum = :new.projnum) IS NULL
)
BEGIN
  INSERT INTO offer_log (offer, status, date)
  VALUES (null, 9, sysdate);
END;

Answer 1

This trigger would do it:

CREATE OR REPLACE TRIGGER trigger_w_subquery
AFTER UPDATE OR INSERT ON project_archiving
FOR EACH ROW WHEN (old.archiving_status <> new.archiving_status
  AND new.archiving_status = 1
)
DECLARE
  l_offer projects.offer%TYPE;
BEGIN
  SELECT offer INTO l_offer 
  FROM projects 
  WHERE projnum = :new.projnum;

  IF l_offer IS NULL THEN
    INSERT INTO offer_log (offer, status, date)
    VALUES (null, 9, sysdate);
  END IF;
END;

I have assumed that the select from projects will always find a row; if not it will raise a NO_DATA_FOUND exception that you may need to handle.

Answer 2

I expect that you want something like

CREATE OR REPLACE TRIGGER trigger_w_subquery
AFTER UPDATE OR INSERT ON project_archiving
FOR EACH ROW 
WHEN (old.archiving_status <> new.archiving_status
  AND new.archiving_status = 1)
DECLARE
  l_offer projects.offer%TYPE;
BEGIN
  SELECT offer 
    INTO l_offer
    FROM projects 
   WHERE projnum = :new.projnum;

  IF( l_offer IS NULL )
  THEN
    INSERT INTO offer_log (offer, status, date)
      VALUES (null, 9, sysdate);
  END IF;
END;

Answer 3

Can you put the condition into the action (between BEGIN and END) instead of in the 'whether it fires'? Yes, it means that the trigger body might be fired more often - but if it gets you around the problem...