我用oracle sql编写了以下脚本：

CREATE OR REPLACE FUNCTION working_time
RETURN INT
IS
diff INT;
sum_ INT;
CURSOR cur IS SELECT * FROM users_;
BEGIN
sum_ := 0;
FOR usr IN cur LOOP
    SELECT SUM(NVL(end_, CURRENT_DATE) - start_)
    INTO diff
    FROM experience
    WHERE user_ = usr.id_;

    sum_ := sum_ + diff;

    IF usr.id_ != 100 THEN
    DBMS_OUTPUT.PUT_LINE('User ' || usr.first_name || ' ' || usr.last_name || ' has worked for ' || diff || ' days.');
    END IF;

    DBMS_OUTPUT.PUT_LINE('return: ' || sum_);
END LOOP;

DBMS_OUTPUT.PUT_LINE('return: ' || sum_);
RETURN sum_;
END working_time;

sum_u1;inside for loop的值打印正确，但在循环外它什么也不打印。这个变量是在for循环之前声明的，所以我不明白发生了什么。有人能解释一下如何修复它以及为什么会发生这种情况吗？
编辑：发布完整代码