Help with writing an UPDATE Trigger

Question

I need to put an update restriction on a field in a table.
For example, in my employee table, I want to disallow updates to telephone number but allow updates to all other fields.
Can someone help me with the coding?
Here is a sample employee table structure.
Table Name: Employee
Date Fields: LName, Fname, SSN, Phone, Address, City, State, Zip
Thanks in advance.

Answer 1

The best way to do this would be to restrict users so they can only update tables through stored procedures and then not prevent updates to phone number in the procedure.
In a trigger, however
CREATE OR REPLACE TRIGGER trg_name
  BEFORE UPDATE ON employee
  FOR EACH ROW
BEGIN
  IF( :new.phone != :old.phone )
  THEN
    RAISE_APPLICATION_ERROR( -20001, 'Updates to phone number are not allowed.' );
  END IF;
END;Note that
- I'm ignoring the possibility that the old or new phone number could be NULL
- You would probably want to declare an exception type in a package and throw that exception to make it easier for the caller to catch.
Justin