1.只是粗略返回相隔的天數
我們知道在Oracle中兩個日期相減返回的就是相差的天數，當然這個天數未必是整數，它會把兩個日期相差的時分秒折算成天數
SQL> SELECT SYSDATE-HIREDATE FROM EMP;
SYSDATE-HIREDATE
----------------
3.12123843
9316.66291
9314.66291
SQL> SELECT SYSDATE-HIREDATE,TRUNC(SYSDATE-HIREDATE) FROM EMP;
SYSDATE-HIREDATE TRUNC(SYSDATE-HIREDATE)
---------------- -----------------------
3.12486111 3
9316.66653 9316
9314.66653 9314
2.返回精確的時間間隔
比如我要得到兩個日期相隔幾天幾小時幾分幾秒，這時就不能用TRUNC來實現了，我們可以用NUMTODSINTERVAL和EXTRACT來實現：
SQL> SELECT EXTRACT(DAY FROM NUMTODSINTERVAL(SYSDATE-HIREDATE,'DAY')) DAY,
2 EXTRACT(HOUR FROM NUMTODSINTERVAL(SYSDATE-HIREDATE,'DAY')) HOUR,
3 EXTRACT(MINUTE FROM NUMTODSINTERVAL(SYSDATE-HIREDATE,'DAY')) MINUTE,
4 NUMTODSINTERVAL(SYSDATE-HIREDATE,'DAY') DETAIL
5 FROM EMP;
DAY HOUR MINUTE DETAIL
---------- ---------- ---------- ------------------------------
3 3 31 +000000003 03:31:54.999999999
9316 16 31 +000009316 16:31:55.000000000
9314 16 31 +000009314 16:31:55.000000000