Primary key not null - sql

Question

I am doing an Sql assignment in which I need to create a few tables. The assignment requires:
Make sure that you define NOT NULL constraints for the PK of each table
I don't get this. When we are defining a key as Primary Key, why should we write NOT NULL separately with it? Doesn't defining a key as PRIMARY KEY itself mean not null and unique?
Please explain!
Edit (copied from below):
CREATE TABLE Faculty(
FacNo char(11) not null,
FacFirstName varchar(30) not null,
FacLastName varchar(30) not null,
FacCity varchar(30) not null,
FacState char(2) not null,
FacZipCode char(10) not null,
FacRank char(4),
FacHireDate date,
FacSalary decimal(10,2),
FacSupervisor char(11),
FacDept char(6),
CONSTRAINT FacultyPK PRIMARY KEY (FacNo));
Is this correct? The FACNO column is not null plus it's also a primary key.

Answer 1

http://www.techonthenet.com/oracle/primary_keys.php
In Oracle, a primary key is a single field or combination of fields
that uniquely defines a record. None of the fields that are part of
the primary key can contain a null value. A table can have only one
primary key.
when you set PK for a table the column will be set to NOT NULL even if you specify it as nullable
-- Create table
create table t_test_pk(
col1 varchar2(5) null
);
SQL> desc t_test_pk
Name Type Nullable Default Comments
---- ----------- -------- ------- --------
COL1 VARCHAR2(5) Y
so... the column is nullable
then we set PK for the table:
SQL> alter table t_test_pk add constraint pk_1 primary key (COL1);
Table altered
and try to insert null into
SQL> insert into t_test_pk values (null);
insert into t_test_pk values (null)
ORA-01400: cannot insert NULL into ("T_TEST_PK"."COL1")
something was changed! check in SqlPlus - the column is not nullable - and get error... we cannot insert null into the column because it was used in PK
SQL> desc t_test_pk;
Name Type Nullable Default Comments
---- ----------- -------- ------- --------
COL1 VARCHAR2(5)
OK... try to set it to nullable
SQL> alter table t_test_pk modify col1 null;
alter table t_test_pk modify col1 null
ORA-01451: column to be modified to NULL cannot be modified to NULL

Answer 2

I imagine the reason your instructor asks for this is to make sure you write DDL that shows your intent clearly. Trusting auto conversion of null to not null does not help readability of DDL so I'd request all DDL to be written such that the create table statement shows the intended nullability of all columns.

Answer 3

Problem solved, with the following query
CREATE TABLE Faculty(
FacNo char(11) not null,
FacFirstName varchar(30) not null,
FacLastName varchar(30) not null,
FacCity varchar(30) not null,
FacState char(2) not null,
FacZipCode char(10) not null,
FacRank char(4),
FacHireDate date,
FacSalary decimal(10,2),
FacSupervisor char(11),
FacDept char(6),
CONSTRAINT FacultyPK PRIMARY KEY (FacNo) );