A primary key is defined as a column or a group of column that their value are always be unique. Normally, NULL value will never be allowed in this column to be part of this column�s records.
EXAMPLE :
Let's say we want to create a table name Users. The PRIMARY KEY will be user_id for that table.
SQL statement is:
CREATE TABLE USERS (
user_id SMALLINT NOT NULL PRIMARY KEY,
user_name VARCHAR(50),
)
OR
CREATE TABLE USERS (
user_id SMALLINT NOT NULL ,
user_name VARCHAR(50),
PRIMARY KEY(user_id)
)