Oracle计算两个时间的差值

上代码：

CREATE OR REPLACE FUNCTION F_GET_DIFF_TIME(START_TIME IN DATE,
                                          END_TIME  IN DATE)
  RETURN VARCHAR2 IS
  DIFF_TIME VARCHAR2(50);
BEGIN
  SELECT TDAY || '天' || THOUR || '时' || TMINUTE || '分' || ROUND((TT - TMINUTE) * 60) || '秒' INTO DIFF_TIME
    FROM (SELECT TDAY,
                THOUR,
                TRUNC((TT - THOUR) * 60) TMINUTE,
                (TT - THOUR) * 60 TT
            FROM (SELECT TDAY,
                        TRUNC((TT - TDAY) * 24) THOUR,
                        (TT - TDAY) * 24 TT
                    FROM (SELECT TO_NUMBER(END_TIME - START_TIME) AS TT,
                                TRUNC(TO_NUMBER(END_TIME - START_TIME)) AS TDAY
                            FROM (SELECT START_TIME, END_TIME FROM dual))));

  RETURN DIFF_TIME;
END;

输出的格式为下图：