Oracle database use of sequences for incrementing values.

We always find a need to use incremental sequence for our table data columns like primary key or other unique keys. Examples of such fields could be id fields like employee id or order sequences for order processing in OLTP databases.

Oracle provides very efficient non blocking way of getting and generating these new sequences with a database object called sequence.

Sequence should be created and used as follows.

CREATE SEQUENCE employee_id_seq
 START WITH     10000   -- Start your sequence with 10000
 INCREMENT BY   1       -- Sequence would be incremented by 1 when nextvalue is fetched from sequence
  NOCYCLE;              -- Oracle sequence would not restart 
                        -- If you want sequence to start over after N then specify maxvalue N clause in the sequence

Full syntax of sequence statement

Once we are done creating sequence object in database next thing is pretty straightforward.
You can use your sequence immediately in insert statement.

INSERT INTO EMPLOYEES (EMPLOYEE_ID , FIRST_NAME,LAST_NAME ) 
VALUES
(employee_id_seq.NEXTVAL , 'PATRICK' , 'HARRIS');

If you are using Oracle Forms or some other tool that generates Inserts for you and if you do not have direct control then consider writing a BEFORE INSERT Trigger that would call the sequnce.nextvalue and provide the next sequence to your insert.
Ex.

CREATE OR REPLACE TRIGGER employees_pre_ins
BEFORE INSERT ON employees
FOR EACH ROW
begin
  SELECT employee_id_seq.NEXTVAL
  INTO   :new.employee_id
  FROM   dual;
END;
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