Add Foreign Key SQL Server

Hello Dev, welcome to this journal article that focuses on how to add foreign keys to SQL Server. In this article, we will cover every aspect of adding foreign keys, from the basics to advanced concepts. Whether you are new to SQL Server or an experienced database administrator, this article will provide you with the knowledge you need to add foreign keys to your database.

What is a Foreign Key?

A foreign key is a column or a combination of columns that connects two tables together. It is used to ensure referential integrity between the tables. In other words, it ensures that the values in the foreign key column(s) in one table match the values in the primary key column(s) in another table.

Let’s take an example. Suppose you have two tables, one for customers and another for orders. The customers table has a primary key column called CustomerID, and the orders table has a foreign key column called CustomerID. The foreign key column in the orders table references the primary key column in the customers table. This ensures that every order in the orders table is associated with a customer in the customers table. If a customer is deleted from the customers table, all the orders associated with that customer will be deleted as well. This is the basic concept of referential integrity.

How to Add Foreign Keys

Step 1: Create Tables

The first step in adding foreign keys is to create the tables. You can use the CREATE TABLE statement for this.

CREATE TABLE Customers (CustomerID INT PRIMARY KEY,FirstName VARCHAR(50),LastName VARCHAR(50))

CREATE TABLE Orders (OrderID INT PRIMARY KEY,OrderDate DATE,CustomerID INT,CONSTRAINT FK_CustomerID FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID))

In this example, we have created two tables, Customers and Orders. The Customers table has three columns, CustomerID, FirstName, and LastName. The CustomerID column is the primary key. The Orders table has three columns, OrderID, OrderDate, and CustomerID. The OrderID column is the primary key, and the CustomerID column is the foreign key that references the CustomerID column in the Customers table. The CONSTRAINT keyword is used to define the foreign key constraint.

Step 2: Insert Data

The next step is to insert data into the tables. You can use the INSERT INTO statement for this.

Step 3: Add Foreign Keys

The final step is to add the foreign key constraint to the foreign key column in the child table. You can use the ALTER TABLE statement for this.

FAQ

What happens if I try to add a foreign key to a column that is not a primary key?

If you try to add a foreign key to a column that is not a primary key, you will get an error message. The foreign key column must reference a primary key column in another table.

Can a table have multiple foreign keys?

Yes, a table can have multiple foreign keys, and each foreign key can reference a different table or the same table.

Can I add a foreign key constraint to an existing table?

Yes, you can add a foreign key constraint to an existing table using the ALTER TABLE statement.

What is the difference between a foreign key and a primary key?

A foreign key is a column or a combination of columns that is used to connect two tables together. It ensures referential integrity between the tables. A primary key is a column or a combination of columns that uniquely identifies each row in a table. It is used to enforce entity integrity.

What happens if I delete a row from the parent table?

If you delete a row from the parent table, all the rows in the child table that reference the deleted row will also be deleted. This is called cascading deletion.

Can I disable a foreign key constraint?

Yes, you can disable a foreign key constraint using the ALTER TABLE statement. This can be useful if you need to temporarily bypass the constraint for some reason.