SQL & PL/SQL

Hi,

I have a table employee with following defination :-

CREATE TABLE EMPLOYEE(

ID NUMBER,

FIRST_NAME VARCHAR2(100),

MIDDLE_NAME VARCHAR2(10),

LAST_NAME VARCHAR2(100),

DOB DATE);

My requirement is that i want to have a not null constraint on all 3 name columns.

Something like

alter table employee modify NVL(FIRST_NAME,NVL(MIDDLE_NAME,LAST_NAME)) not null;

How can i achieve this?