Foreign Key constraint in Sql server

Introduction: In this article today ia m going to explain foreign Key constraint in sql server

Description:

A foreign key Constraint is used to link two tables together. A Foreign key in one table refers to primary key of another table.  It can accept multiple null and duplicate values.

Example:

Create a table with foreign key constraint:

Create table Customers

(

CustomerId int Primary key,

Name varchar(50),

Address Varchar(150),

Phone_Number int

)

Create Table Orders

(

OrderId int Primary key,

OrderNumber int not null,

Customer_Id_Fk int,

constraint Customer_Id_Fk FOREIGN KEY (Customer_Id_Fk) REFERENCES Customers(CustomerId)

)

In this example I have create a two tables, one Customers which have the customers information and another Orders which have stored the detail of order placed by cuntomers. Customer_Id_Fk of  Orders table is forgien key  linked to  CustomerId of Customers table.

Alter a table with foreign key constraint:

ALTER TABLE Orders

add constraint Customer_Id_Fk FOREIGN KEY (Customer_Id_Fk) REFERENCES Customers(CustomerId)

Drop a table with foreign key constraint:

ALTER TABLE Orders

DROP constraint Customer_Id_Fk