sql - 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. Note: This trigger is not intended to address the average GPA of any given student, but rather it should address the average grade for all grades assigned in a particular class.

Here's the schema:

Student-schema =(studentnum, name, standing, gpa, major)
Class-schema = (schedulenum, semester, department, classnum, days, time, place, enrollment)
Instructor-schema = (name, department, office)
Teaches-schema = (name, schedulenum, semester)
Taking-schema = (studentnum, schedulenum, semester, grade)

I'm having a terrible time with these triggers, but here's my attempt to make this work:

CREATE OR REPLACE TRIGGER stopChange
    AFTER 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 AVG(grade)
    INTO grd_avg
    FROM taking
    WHERE studentnum = :new.studentnum
    AND schedulenum = :new.schedulenum
    AND semester = :new.semester;

    IF grd_avg < 2.5 THEN
        UPDATE taking
        SET grade = :old.grade
        WHERE studentnum = :old.studentnum
        AND schedulenum = :old.schedulenum
        AND semester = :old.semester;
    END IF;

END;   
/

I'm obviously doing something wrong because when I then go to update or delete a tuple, I get the error:

ERROR at line 1:
ORA-04091: table TAKING is mutating, trigger/function may not see it
ORA-06512: at "STOPCHANGE", line 6
ORA-04088: error during execution of trigger 'STOPCHANGE'

Any advice? I'm using Oracle.

Answer 1

First you need to read about triggers, mutating table error and compound triggers: http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#LNPLS2005

Your trigger is AFTER UPDATE OR INSERT OR DELETE. Means if you run UPDATE OR INSERT OR DELETE statements on this table, the trigger will fire. But you are trying to update the same table again inside your trigger, which is compl. wrong. This this is why you are getting the error. You cannot modify the same table the trigger is firing on. The purpose of trigger is to fire automatically when table is updated, inserted or deleted in your case. What you need is some procedure, not trigger.