NEXTVAL and CURRVAL Pseudocolumns

• NEXTVAL : The NEXTVAL pseudocolumns is used to extract successive sequence numbers from a specified sequence.
  sequence.NEXTVAL
  
• CURRVAL : The CURRVAL pseudocolumn is used to refer to a sequence number that the current user has just generated.
  

After you generated a sequence object, you have to call NEXTVAL first because the initiatied pointer of the sequence object is pointing the head of the object which has NULL.