Creating Trigger long after table creation

I would like to find out what the consquence is if you want to create a sequence after a table has been created and quite a bit of data already been inserted.

( this is because PEAR's DataObject's insert() method sometimes skips incremental IDs )

So here is an example to achieve this, but is this the correct way to do if after the amount of time has passed?

Table definition:

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;

Trigger definition:

CREATE OR REPLACE TRIGGER dept_bir 
BEFORE INSERT ON departments 
FOR EACH ROW

BEGIN
  SELECT dept_seq.NEXTVAL
  INTO   :new.id
  FROM   dual;
END;

Solution

If you mean that you already have datas with ID field inserted without using the trigger, the only thing you'll have to check is that the "start" of your sequence = at least the max existing ID + 1

CREATE SEQUENCE dept_seq
 START WITH     2503
 INCREMENT BY   1

Then it should be perfectly fine.