Table is mutating, trigger/function may not see it (stopping an average grade from dropping below 2.5)

Question

Here\'s the problem:

Create a trigger that prevents any change to the taking relation that would drop the overall average grade in any particular class below 2.5. No

Answer 1

I think you can fix this by rewriting this as a before trigger, rather than an after trigger. However, this might be a little complicated for inserts and deletes. The idea is:

CREATE OR REPLACE TRIGGER stopChange
    BEFORE UPDATE OR INSERT OR DELETE ON taking
    REFERENCING OLD AS old
    NEW AS new
    FOR EACH ROW
DECLARE

grd_avg taking.grade%TYPE;

BEGIN
    SELECT (SUM(grade) - oldgrade + new.grade) / count(*)
    INTO grd_avg
    FROM taking
    WHERE studentnum = :new.studentnum
    AND schedulenum = :new.schedulenum
    AND semester = :new.semester;

    IF grd_avg < 2.5 THEN
        new.grade = old.grade
    END IF;
END;