BEGIN
    DECLARE loop_done INT DEFAULT 0;
    DECLARE current_user_id INT;
    DECLARE current_todo_id INT;
    DECLARE todo_position INT DEFAULT 0;
    DECLARE cur_users CURSOR FOR
        SELECT id FROM users;
    DECLARE cur_todos CURSOR FOR
        SELECT id FROM todo WHERE user_id = @uid;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET loop_done = 1;

    OPEN cur_users;
    loop_users: LOOP

        FETCH cur_users INTO current_user_id;

        IF loop_done = 1 THEN
            SET loop_done = 0;
            LEAVE loop_users;
        END IF;

        SET @uid = current_user_id;
        SET todo_position = 0;

        OPEN cur_todos;

        loop_todos: LOOP
            FETCH cur_todos INTO current_todo_id;
            IF loop_done = 1 THEN
                SET loop_done = 0;
                SET todo_position = 0;
                LEAVE loop_todos;
            END IF;

            UPDATE todo SET position = todo_position WHERE user_id = @uid;
            SET todo_position = todo_position + 1;

        END LOOP loop_todos;

        CLOSE cur_todos;

    END LOOP loop_users;
END$$

+----------+
| position |
+----------+
|        3 |
|        3 |
|        3 |
|        3 |
|        5 |
|        5 |
|        5 |
|        5 |
|        5 |
+----------+

+----------+
| position |
+----------+
|        0 |
|        1 |
|        2 |
|        3 |
|        0 |
|        1 |
|        2 |
|        3 |
|        4 |
+----------+