AutoNumber And Identity Functionality in Oracle Databases (Pre 12c)

Oracle Database 12c has introduced two new features that make the trigger-based solution for populating numeric ID columns redundant. If you are using Oracle 12c onward, you should consider one of the following options.

• Identity Columns in Oracle Database 12c Release 1 (12.1)
• DEFAULT Values Using Sequences in Oracle Database 12c Release 1 (12.1)

If you are using a version of the database prior to Oracle 12c, this article describes how to implement AutoNumber/Identity type functionality in Oracle.
Developers who are used to AutoNumber columns in MS Access or Identity columns in SQL Server often complain when they have to manually populate primary key columns using sequences in Oracle. This type of functionality is easily implemented in Oracle using triggers.
Create a table with a suitable primary key column and a sequence to support it.

CREATE TABLE departments (
  ID           NUMBER(10)    NOT NULL,
  DESCRIPTION  VARCHAR2(50)  NOT NULL);

ALTER TABLE departments ADD (
  CONSTRAINT dept_pk PRIMARY KEY (ID));

CREATE SEQUENCE dept_seq;

Create a trigger to populate the ID column if it's not specified in the insert.

CREATE OR REPLACE TRIGGER dept_bir 
BEFORE INSERT ON departments 
FOR EACH ROW
WHEN (new.id IS NULL)
BEGIN
  SELECT dept_seq.NEXTVAL
  INTO   :new.id
  FROM   dual;
END;
/

Test it using the automatic and manual population methods.

SQL> INSERT INTO departments (description)
  2  VALUES ('Development');

1 row created.

SQL> SELECT * FROM departments;

        ID DESCRIPTION
---------- --------------------------------------------------
         1 Development

1 row selected.

SQL> INSERT INTO departments (id, description)
  2  VALUES (dept_seq.NEXTVAL, 'Accounting');

1 row created.

SQL> SELECT * FROM departments;

        ID DESCRIPTION
---------- --------------------------------------------------
         1 Development
         2 Accounting

2 rows selected.

SQL>

The trigger can be modified to give slightly different results. If the insert trigger needs to perform more functionality than this one task you may wish to do something like the following.

CREATE OR REPLACE TRIGGER dept_bir 
BEFORE INSERT ON departments 
FOR EACH ROW
BEGIN
  SELECT NVL(:new.id, dept_seq.NEXTVAL)
  INTO   :new.id
  FROM   dual;
  
  -- Do more processing here.
END;
/

To overwrite any values passed in you should do the following.

CREATE OR REPLACE TRIGGER dept_bir 
BEFORE INSERT ON departments 
FOR EACH ROW
BEGIN
  SELECT dept_seq.NEXTVAL
  INTO   :new.id
  FROM   dual;
END;
/

To error if a value is passed in you should do the following.

CREATE OR REPLACE TRIGGER dept_bir 
BEFORE INSERT ON departments 
FOR EACH ROW
BEGIN
  IF :new.id IS NOT NULL THEN
    RAISE_APPLICATION_ERROR(-20000, 'ID cannot be specified');
  ELSE
    SELECT dept_seq.NEXTVAL
    INTO   :new.id
    FROM   dual;
  END IF;
END;
/