SQL Query to Add Foreign Key Constraints Using ALTER Command

In this article, we will look into how you can add a foreign key constraint using the ALTER command in SQL. For this article, we will be using Microsoft SQL Server as our database.

But before we start let’s take a look at the below terminologies:

• Primary key: It is used to uniquely identify the records in the table. It is mainly used in Relational Database. A Primary must be unique.
• Foreign Key: It is used to link tables together. This is also called referencing. It is used for referring to a Primary Key of another table.

Create a Database:

We can create a Database using the command:

Syntax: CREATE DATABASE DATABASE_NAME;

So let’s create a geeks database as shown below:

CREATE DATABASE geeks;

Using Database:

Use the below command to use the geeks database:

use geeks;

Adding table into Database:-

To add a table into the database we use the below command:

Syntax:- CREATE TABLE table_name (Attribute_name datatype...);

So, let’s create a geeks table within the geeks database as shown below:

CREATE TABLE geeks(id int , name varchar(20));

Here Table Added Successfully.

Inserting values into Tables:

For inserting records into the table we can use the below command:

Syntax: INSERT INTO table_name(column1,
                column2,
                column 3,.....)
                 VALUES( value1,
                value2,
                value3,.....);

So let’s add some records in the geeks table:

 INSERT INTO geeks(id,name) VALUES (1,"teja");

Creating Primary Key Element in a Table:

To have a Foreign Key in a Table we must have a Primary Key. To create a Primary we use the below command:

Syntax: CREATE TABLE table_name (Attribute_name  datatype PRIMARY_KEY);

Now let’s create a primary key:

CREATE TABLE emp (id int NOT NULL PRIMARY KEY,name varchar(20))

Now to add a Foreign Key we have to create a new table by the following:

CREATE TABLE  student(
id int ,
name varchar(20),
Emp_id int REFERENCES emp(id));

Alter a Table and ADD Foreign Key:

So if you already created the table student, and now you wish to add Foreign Key you can use the below command to change that:

ALTER TABLE dbo.student 
add  constraint Fk_empid foreign key(emp_id)
references dbo.emp(id);

At this point, we have successfully achieved our goal.