Primary key constraint in sql server

Introduction: In this article today I am going to explain Primary key constraint in sql server

Description:

A primary key constraint uniquely identifies each record and prevents the duplicate values for a column or columns in a database table. Primary key column can't have null values. Each table can have only one Primary key and it creates clustered index on the column or columns.

Example to create a table with primary key constraint:

Create table Book

(

Id int NOT NULL IDENTITY (1, 1) CONSTRAINT Id PRIMARY KEY(Id),

Book_Name varchar(50),

Book_price int,

Publisher varchar(50)

)

In this example I create a table Book and set Id its primary key constraint.

Example to alter the table and create primary key constraint:

Alter table Book

Add CONSTRAINT Id PRIMARY KEY(Id)

Example to delete/drop the primary key constraint:

Alter table Book

drop CONSTRAINT id

Note: We can use two or more columns as a primary key, they are called composite key.

Example of composite key:

Create table Book

(

Id int NOT NULL IDENTITY (1, 1),

Book_Name varchar(50),

Book_price int,

Publisher varchar(50)

CONSTRAINT Id PRIMARY KEY(Id,Book_Name)

)

In this example Id and Book_Name must be unique for the table