How to Use primary key in column level and table level

Primary key combination of unique and not null.that means column can't allow null vales as well as duplicates.
A table can have only one primary key.

if you want create primary key for multiple columns we need to use table level constraint.

column level

create table student(sno number primary key,sname varchar2(10),sclass number not null);

Table level

create table student(sno number,
                     sname varchar2(10)
                     sclass number not null
                     primary key(sname,sclass)
                    );