PL/SQL trigger does not change value of table attribute - plsql

Question

this is a follow up question from my previous post:
PL/SQL Triggers with aggregate function
I have a trigger named updategpa that updates the gpa inside the student table whenever grades get changed in the course table.
This trigger gets called as a result of 'updateGrades' trigger that updates the course table when assignments table receives new data.
Problem: gpa is 0 before and after the trigger is called?
Could anyone say what is wrong with my trigger?
drop table courses;
drop table student;
drop table assignments;
create table student (sid integer, sname char(10), saddress char(10), gpa integer);
create table courses (sid integer, cid integer, cgrade integer);
create table assignments ( sid integer, cid integer, aid integer, agrade integer);
insert into student (sid, sname, saddress, gpa) values (1, 'Mike', 'Brighton', 0);
insert into courses (sid, cid, cgrade) values(1, 2000, 0);
insert into assignments values(1, 2000, 1, 70);
insert into assignments values(1, 2000, 2, 80);
create or replace trigger updateGrades before insert or update or delete of agrade on assignments for each row
begin
if inserting then
update courses set cgrade = :new.agrade where cid = :new.cid and sid = :new.sid;
elsif updating then
update courses set cgrade = :old.agrade + (:new.agrade - :old.agrade)
where cid = :new.cid and sid = :new.sid;
elsif deleting then
delete from courses where cid = :new.cid and sid = :new.sid;
end if;
end;
/
show errors;
create or replace trigger updategpa after insert or update or delete of cgrade on courses for each row
begin
if inserting then
update student set gpa =
(select avg(cgrade) from courses inner join student on courses.sid = student.sid
)
where sid = 1;
elsif inserting then
update student set gpa =
(select avg(cgrade) from courses inner join student on courses.sid = student.sid
)
where sid = 1;
elsif deleting then
delete from student where sid = :new.sid;
end if;
end;
/
show errors;
select * from courses;
select * from student;
select * from assignments;
update assignments set agrade = agrade + 5;
select * from courses;
select * from student;
select * from assignments;
console output:
SQL> select * from student;
SID SNAME SADDRESS GPA
---------- ---------- ---------- ----------
1 Mike Brighton 0
SQL> select * from assignments;
SID CID AID AGRADE
---------- ---------- ---------- ----------
1 2000 1 70
1 2000 2 80
SQL>
SQL> update assignments set agrade = agrade + 5;
2 rows updated.
SQL>
SQL> select * from courses;
SID CID CGRADE
---------- ---------- ----------
1 2000 85
SQL> select * from student;
SID SNAME SADDRESS GPA
---------- ---------- ---------- ----------
1 Mike Brighton 0
SQL> select * from assignments;
SID CID AID AGRADE
---------- ---------- ---------- ----------
1 2000 1 75
1 2000 2 85

Answer 1

The root of the problem is your updategpa trigger needs to track every change to COURSES at the row level, then aggregate those changes to maintain a running total for each student. This is a good use case for a compound trigger. Find out more.
create or replace trigger updategpa
for update or update or insert on courses compound trigger
type cid_nt is table of courses%rowtype;
upd_recs cid_nt;
before statement is
begin
upd_recs := cid_nt();
end before statement;
before each row is
begin
upd_recs.extend();
if inserting or updating then
upd_recs(upd_recs.count()).sid := :new.sid;
else
upd_recs(upd_recs.count()).sid := :old.sid;
end if;
end before each row;
after each row is
begin
null;
end after each row;
after statement is
begin
for idx in 1 .. upd_recs.count() loop
update student
set gpa = (select avg(cgrade)
from courses
where courses.sid = student.sid )
where sid = upd_recs(idx).sid;
end loop;
end after statement;
end;
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