PRIMARY KEY

The PRIMARY KEY constraints uniquely identifies each record in a database table. Primary keys must contain UNIQUE values. A primary key column cannot contain NULL values. Most a tables should have a primary key, and each table can have only ONE primary key.

Example:-

CREATE TABLE student 
(
Name varchar (30),
Roll integer (5) NOT NULL PRIMARY KEY,
Mobile_no integer (10)
);

CREATE TABLE student 
(
Name varchar (30),
Roll integer (5) NOT NULL,
Mobile_no integer (10),
PRIMARY KEY (Roll)
);