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´┐Żs 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)
)