Oracle trigger to create an autonumber

Question

I have never created a trigger in Oracle before so I am looking for some direction.

I would like to create a trigger that increments an ID by one if the ID isnt in the i

Answer 1

I would recommend to code this trigger with a condition on the trigger itself, not in the sql block.

CREATE OR REPLACE TRIGGER your_trigger
BEFORE INSERT ON your_table
REFERENCING NEW AS NEW OLD AS OLD
FOR EACH ROW
WHEN (new.id IS NULL)
  BEGIN
    SELECT your_sequence.nextval
    INTO :new.id
    FROM dual;
  END;
/

With this solution the trigger is only executed if the condition matches (id is null).

Otherwise the trigger is always executed and the block checks if id is null. The DB must execute the SQL block which does nothing on not null values.