SQL Query to Add Foreign Key Constraints Using ALTER Command

In relational databases, a Foreign Key Constraint is a fundamental concept that defines a relationship between two tables. It ensures that the value of one or more columns in a child table corresponds to the value in a primary key or unique key in a parent table.

In this article, we will look into how we can add a foreign key constraint using the ALTER command in SQL.

Foreign Key Constraint

• A Foreign Key Constraint in a relational database is a rule that establishes a relationship between two tables.
  
• It ensures that the values in a column (or a group of columns) of one table (called the child table) must match the values in a column (or a group of columns) in another table (called the parent table).
  
• The column(s) in the parent table is referred to as the primary key or unique key, and the column(s) in the child table that references this primary key is the foreign key.

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;

Output:

Using Database:

Use the below command to use the geeks database:

use geeks;

Output:

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));

Output:

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");

Output:

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))

Output:

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));

Output:

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);

Output:

At this point, we have successfully achieved our goal.

Conclusion

The Foreign Key Constraint is a crucial component in relational database management. It helps ensure the integrity and accuracy of your data by linking related records between different tables. By defining these relationships, you can prevent the entry of invalid data and establish clear connections between tables for more structured data management.