Primary key and Unique index

SQL> create table t1(id1 char(2),id2 char(2),id3 char(2));

Table created.

SQL> desc t1
 Name					   Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID1						    CHAR(2)
 ID2						    CHAR(2)
 ID3						    CHAR(2)

SQL> insert into t1 values('a',null,'a');

1 row created.

SQL> commit;

Commit complete.

SQL> select * from t1;

ID ID ID
-- -- --
a     a

SQL> alter table t1
  add constraint PK_EMP primary key (id1,id2,id3);  2
  add constraint PK_EMP primary key (id1,id2,id3)
                                         *
ERROR at line 2:
ORA-01449: column contains NULL values; cannot alter to NOT NULL

SQL> create unique index t1_idx1 on t1(id1,id2,id3); 

Index created.

SQL> insert into t1 values('a',null,'a');
insert into t1 values('a',null,'a')
*
ERROR at line 1:
ORA-00001: unique constraint (SYS.T1_IDX1) violated