How to write a 'Before Delete' trigger in SQL - mysql

Question

I am currently working on a homework project where we are creating triggers user plpgsql
I've managed to figure out the trigger for update/insert but I'm having trouble with writing
a delete one and feel like im going round in circles with the online documentation.
The two tables we're given are:
MAJOR(mCode, name) with primary key mCode.
STUDENT(sId, firstName, lastName, mCode, pointsEarned) with primary key {sId} and foreign key mCode
The requirement is basically that a major cannot be deleted or changed if a student has chosen the major.
(I've already implemented the update/insert trigger)
I'm 99% sure my create trigger will just be:
CREATE TRIGGER major_delete
BEFORE UPDATE OR INSERT ON student
FOR EACH ROW EXECUTE PROCEDURE major_delete_trigger();
And I have come up with two basic ideas for the function (based off lectures notes):
CREATE OR REPLACE FUNCTION major_delete_trigger()
RETURNS trigger AS $$
DECLARE m RECORD;
BEGIN
SELECT * INTO m FROM major WHERE OLD.mcode = mcode;
IF NOT FOUND THEN RETURN OLD;
ELSE
DELETE FROM major WHERE mcode = OLD.mcode;
RETURN NULL;
END IF;
END;
$$ LANGUAGE 'plpgsql';
Which does nothing at all - when running a test on my database deletes that should be failing still succeed.
Secondly:
CREATE OR REPLACE FUNCTION major_delete_trigger()
RETURNS trigger AS $$
BEGIN
IF (TG_OP = 'DELETE') THEN
DELETE FROM major WHERE mcode=OLD.mcode;
IF NOT FOUND THEN RETURN NULL; END IF;
END IF;
END;
$$ LANGUAGE 'plpgsql';
Which executes the line 'DELETE FROM major WHERE mcode = OLD.mcode;' until there is no room left on the stack.
When doing my update function I found I was just looking at it as if it was really complicated so I'm not sure if this will be the same.
Any ideas or documentation that could help will be much appreciated!