SQL*Plus - PK Question

I have come across many examples in sql*plus with regards to using 'not nulls' in conjunction with primarys keys. This bothers me because from my reading when a primary key is defined for an attributes(s) of a table, it assumes that those values cannot contain null values, but yet i see this:

create table Example
(

    sid char(5) not null,
    ....
    constraint pk_sid primary key (sid)
);

why need to use not null when primary key implicitly imposes that contraint?

Any help appreciated. Received on Tue Apr 23 2002 - 02:02:14 CDT