Oracle Creating Primary Keys

We just created this table:

CREATE TABLE DEPENDENTS
 ( "ID" NUMBER(6,0)not null,
 "FIRSTNAME" VARCHAR2(20),
 LASTNAME VARCHAR2(20),
 BIRTHDATE DATE,
 RELATION VARCHAR2 (25),
 GENDER VARCHAR2(1),
 BENEFITS CLOB,
 RELATIVEID VARCHAR2(10) );

So we need to add a primary key linked to ID field doing:

ALTER TABLE DEPENDENTS add CONSTRAINT DEPENDENTS_PK PRIMARY KEY(ID) ;
table altered.

We also want to create a data sequence for populating the ID field:

CREATE SEQUENCE DEPENDENTS_SEQ MAXVALUE 999 MINVALUE 1;

Now we can use this sequence to enter ID value "by default":

create or replace trigger bi_dependents 
before insert on dependents 
for each row 
begin
  select DEPENDENTS_SEQ.nextval into :NEW.ID from dual;
end;