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;