Primary Key On Tables

Primary Key

    The Primary Key Constraint Uniquely identifies each Record in a Table. Primary key Must Contain Unique Values and it won't accept  NULL values.

                                    PRIMARY KEY = UNIQUE + NOT NULL

Each table should have a primary key, and each table can have only one Primary Key..

Syntax For Creating Primary Key..

Method (1) :-

CREATE TABLE SAMPLE

(

   id int identity(1,1) Primary Key,

   First_Name varchar(10) not null,

   Last_Name varchar(10) not null

)

Method (2) :-

CREATE TABLE SAMPLE

(

   id int identity(1,1),

   First_Name varchar(10) not null,

   Last_Name varchar(10) not null

   primary key (id)

)

Method (3):-

CREATE TABLE SAMPLE
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)