create table xyz
(
f1 number not null,
f2 varchar2(20) not null,
f3 number not null,
f4 varchar2(50),
f5 number(5),
primary key (f1, f2, f3) 
);

select * from xyz where f1 = 5 and f2 = 'adcf' and f3 = 454;

create table abc
(
pk number(3) not null,
f1 number not null,
f2 varchar2(20) not null,
f3 number not null,
f4 varchar2(50),
f5 number(5),
primary key (pk) ,
unique (f1,f2,f3)
);

select * from abc where f1 = 5 and f2 = 'adcf' and f3 = 454;

create table xyz
(
f1 number not null,
f2 varchar2(20) not null,
f3 number not null,
f4 varchar2(50),
f5 number(5),
primary key (f1, f2, f3) 
);



create table abc
(
pk number(3) not null,
f1 number not null,
f2 varchar2(20) not null,
f3 number not null,
f4 varchar2(50),
f5 number(5),
primary key (pk) ,
unique (f1,f2,f3)
);

create table ref_fgh
(
pk number(2) primary key,
text varchar2(20),
fk_abc_pk,
FOREIGN KEY (fk_abc_pk) REFERENCES abc
);

select ref_fgh.*, f1,f2,f3
from ref_fgh, abc
where fk_abc_pk = abc.pk;

create table ref_fgh2
(
pk number(2) primary key,
text varchar2(20),
fk_xyz_f1,
fk_xyz_f2,
fk_xyz_f3,
FOREIGN KEY (fk_xyz_f1,fk_xyz_f2,fk_xyz_f3) REFERENCES xyz
);

select * from ref_fgh2;

SQL> create table a
  2  (
  3     a integer primary key
  4  )
  5  /

Table created.

SQL> create table b
  2  (
  3     b integer unique
  4  )
  5  /

Table created.

SQL> insert into b values (null);

1 row created.

SQL> insert into a values (null);
insert into a values (null)
                      *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("KEVIN"."A"."A")