Primary key

        
Syntax:

CREATE TABLE table_name
(column1 datatype null/not null,
column2 datatype null/not null,
...
 PRIMARY KEY (column1, column2, . column_n)
);

 example:

CREATE TABLE supplier

( supplier_id numeric(10) not null,

 supplier_name varchar2(50) not null,

 contact_name varchar2(50), 

  PRIMARY KEY (supplier_id)

);

Using an ALTER TABLE statement

 Syntax:

         ALTER TABLE table_name
         add PRIMARY KEY (column1, column2, ... column_n);

 Example:

ALTER TABLE supplier
add CONSTRAINT supplier_pk PRIMARY KEY (supplier_id);