Detta fungerade perfekt.
CREATE OR REPLACE TRIGGER checkDuration
BEFORE INSERT OR UPDATE on offering
FOR EACH ROW
DECLARE
isFound NUMBER;
BEGIN
SELECT 1 INTO isFound FROM DUAL WHERE EXISTS (
SELECT * FROM Course c
WHERE c.courseId = :new.courseId AND c.duration = 5);
IF EXTRACT(MONTH FROM :new.startDate) = 12
THEN RAISE_APPLICATION_ERROR(-20001, 'Courses of five days duration cannot be run in December');
END IF;
EXCEPTION
WHEN NO_DATA_FOUND THEN
NULL;
END;