[Oracle] How to Create Before Insert Trigger

Requirement
Here has one of my table schema, and the first column, SEQ_NO, will be assigned serial number which value will be get from SE_FMS_CF0006 sequence.

I hope I do not need get the sequence value manually and assign to SEQ_NO column manually.

Column	Type	Size	Nulls	Default	Comments
SEQ_NO	number	20			流水號SELECT SE_FMS_CF0006.NEXTVAL FROM DUAL
AGE	varchar2	7	√	null	資料提供機構代碼
FILE_TYPE	varchar2	8	√	null	傳輸檔案識別
PDATE	varchar2	7			資料日期
FYR	varchar2	3	√	null	會計年度
ACC	varchar2	11			科目代碼
ACC_APPENDIX	varchar2	3			科目代碼附加碼
AMOUNT	number	15,2	√	0	金額
UPDATE_DATE	timestamp(6)	11,6	√	now	更新日期
USER_ID	varchar2	20	√	null	異動者

Solution
We can make good use of trigger to fulfill this requirement, you can check the before insert trigger. 
As you do insert, you can skip SEQ_NO, this trigger will get the serial number before you do insert.

CREATE OR REPLACE TRIGGER AP_PSR.TR_BI_CF0006  
  BEFORE INSERT ON AP_PSR.CF0006  FOR EACH ROW  
 BEGIN  
  IF :NEW.SEQ_NO IS NULL THEN  
   SELECT SE_FMS_CF0006.NEXTVAL INTO :NEW.SEQ_NO FROM DUAL;  
  END IF;  
 END;  

Demo
Assume I would like to execute the insert statement, I only provide values to mandatory columns except SEQ_NO.

INSERT INTO cf0006(pdate, acc, acc_appendix)
VALUES('1030401', '123', '123');

Then execute the select statement:

SELECT * FROM cf0006;

See.... I did not assign any value to SEQ_NO, its value assigned by TR_BI_CF0006  trigger automatically.

Reference
[1] http://www.techonthenet.com/oracle/triggers/before_insert.php