示例：创建独立的函数。

    1. delimiter /
    2. CREATE OR REPLACE FUNCTION f_maxid_of_t1 RETURN number
    3. IS
    4.     i_id    number := -1;
    5. BEGIN
    6.     SELECT max(id) INTO i_id FROM t1 ;
    7.     RETURN i_id;
    8. EXCEPTION
    9.     WHEN OTHERS THEN 
    10.         RETURN NULL;
    11. END;
    12. /
    13. delimiter ;

    执行函数。

    1. obclient> select * from t1;
    2. +-------+------+---------------------+
    3. | ID    | NAME | GMT_CREATE          |
    4. +-------+------+---------------------+
    5. | 10000 | A    | 2020-04-02 18:37:24 |
    6. | 10001 | B    | 2020-04-02 18:37:41 |
    7. | 10003 | C    | 2020-04-02 18:45:07 |
    8. | 10004 | D    | 2020-04-02 18:45:08 |
    9. | 10005 | E    | 2020-04-02 18:45:10 |
    10. +-------+------+---------------------+
    11. 5 rows in set (0.00 sec)
    13. obclient> select f_maxid_of_t1() from dual;
    14. +-----------------+
    15. | F_MAXID_OF_T1() |
    16. +-----------------+
    17. |           10005 |
    18. +-----------------+
    19. 1 row in set (0.03 sec)
    21. obclient> truncate table t1;
    22. Query OK, 0 rows affected (0.11 sec)
    24. obclient> select f_maxid_of_t1() from dual;
    25. +-----------------+
    26. | F_MAXID_OF_T1() |
    27. +-----------------+
    28. |            NULL |
    29. +-----------------+
    30. 1 row in set (0.02 sec)