MySQL cursor avg() fetch null - mysql

Question

I have a computed column in cursor select query.
drop procedure if exists update_avg;
delimiter $$
create procedure update_avg()
BEGIN
declare score decimal(9,4);
declare id varchar(5);
declare done bool default false;
declare c_update cursor for
select stu_id, avg(score) from chooses group by stu_id;
declare continue HANDLER for not found set done = true;
open c_update;
fetch c_update into id, score;
select id, score; -- test purpose
while(not done) do
update student set average_score = score where student_id = id;
fetch c_update into id, score;
end while;
close c_update;
END
delimiter ;
call update_avg();
When I execute this query it works fine:
select stu_id, avg(score) from chooses group by stu_id;
|stu_id|avg(score)|
|------|----------|
|1 | 73.5000|
|10 | 93.0000|
|11 | 53.0000|
...
And when I call update_avg(); output:
|id|score|
|--|-----|
|1 |NULL |
My question is why this cursor cannot fetch avg(score) from select query and how to fix this.

Answer 1

This operation does not require cursor and could be done using correlated subquery:
update student
set average_score = (SELECT AVG(score)
FROM chooses
WHERE chooses.stu_id = student.student_id)
-- WHERE EXISTS (SELECT 1 FROM chooses WHERE chooses.stu_id = student.student_id)
-- this condition is to avoid updating rows
-- that do not have corresponding rows in chooses table