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