How to properly write a cursor inside a function?

I require your help because I'm kind of lost. I tried to look over google for cursors in functions and references about those but I can't seem to quite wrap my head the concept. I have a homework where I need to :

Create a functions in which I use cursors
The entry is the percentage of the increase in salary (ex: 5% is 0.05)
The functions need to update the salary with the new increased price
Show the new average salary
And deal with possible errors

This is what i've made so far...

Can you help me out with the problem with hint or at least simple examples/ressources of cursors in functions so get I can start get a better hold of the concept.

DROP FUNCTION IF EXISTS Program;
DELIMITER //
CREATE FUNCTION Program(IncSal DECIMAL(3,2))
RETURNS DECIMAL(8,2)
DETERMINISTIC
    BEGIN
        DECLARE Salaire DECIMAL (8,2);
        DECLARE end BOOLEAN DEFAULT FALSE;
        DECLARE CurSalary CURSOR FOR
            SELECT Sala_EMPL
                        FROM EMPLOYE;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET end = TRUE;      

        OPEN CurSalary;
            loop_cursor: LOOP
                IF end THEN 
                    LEAVE loop_cursor; 
                END IF;
                UPDATE EMPLOYE
                SET Sala_EMPL=(Sala_EMPL*IncSal);
            END LOOP;
        CLOSE CurSalary;

        RETURN AVG(CurSalary);
    END; 
    //
    DELIMITER ;

Solution

If you use a cursor you have to FETCH from the cursor (please review https://dev.mysql.com/doc/refman/8.0/en/fetch.html) without a fetch the loop is infinite. AVG is an aggregate function , you cannot AVG over a cursor (you need a select..from). Your update statement will apply to all employees every time you iterate a cursor loop (logically absurd). Personally I would never use a function because what you are trying to do is not really appropriate in a function (i would use a procedure).

an example of a 'working' but wrong function

DROP FUNCTION IF EXISTS f;
DELIMITER //
CREATE FUNCTION f(IncSal DECIMAL(3,2))
RETURNS DECIMAL(8,2)
DETERMINISTIC
    BEGIN
        DECLARE Salaire DECIMAL (8,2);
        DECLARE end BOOLEAN DEFAULT FALSE;
        DECLARE CurSalary CURSOR FOR
        SELECT salary FROM EMPLOYEe;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET end = TRUE;      

        OPEN CurSalary;
            loop_cursor: LOOP
                 fetch cursalary into salaire;
                IF end THEN 
                    LEAVE loop_cursor; 
                END IF;
                UPDATE EMPLOYEe
                            SET Salary=salary + (salary*incsal);
            END LOOP;
        CLOSE CurSalary;

        #RETURN AVG(CurSalary);
        return (select avg(salary) from employee);
    END; 
    //
    DELIMITER ;

MariaDB [sandbox]> select * from employee;
+------------+-----------+--------------+--------+
| employeeid | firstname | departmentid | salary |
+------------+-----------+--------------+--------+
|          1 | aaa       |            1 |  10000 |
|          2 | bbb       |            2 |  10000 |
|          3 | ccc       |            3 |  10000 |
+------------+-----------+--------------+--------+
3 rows in set (0.00 sec)

MariaDB [sandbox]>
MariaDB [sandbox]>     select f(.05);
+----------+
| f(.05)   |
+----------+
| 11576.00 |
+----------+
1 row in set (0.00 sec)

MariaDB [sandbox]>
MariaDB [sandbox]>     select * from employee;
+------------+-----------+--------------+--------+
| employeeid | firstname | departmentid | salary |
+------------+-----------+--------------+--------+
|          1 | aaa       |            1 |  11576 |
|          2 | bbb       |            2 |  11576 |
|          3 | ccc       |            3 |  11576 |
+------------+-----------+--------------+--------+
3 rows in set (0.00 sec)