SQL Foreign Key

SQL Foreign Key constraint is used to implement the relationship between SQL tables. In order to normalize the data, we split the tables using this Foreign key relation. For example, Employee table has Employee ID, and Orders table has Orders information along with Emp ID. Remember, this Emp ID holds Foreign Key relationship with Employee ID column in Employee table.

Before we get into the example, following are the list of points you should remember:

• You can link a Foreign key column with a Primary Key constraint in another column, or a Unique Constraint column in another column.
• Values you enter in the Foreign key column should match with the referenced column otherwise, it will throw an error.
• You can define the foreign key relationship with tables that are present within the same database and same server. You can use Triggers to provide the cross-referential integrity.
• SQL allows you to define the foreign key relationship with another column present within the same table. This type of relationship is self-reference.
• You can’t delete a table that has the foreign key relationship

For this demonstration we are using the Department table.

Create SQL Foreign Key

We can create a SQL Server Foreign key using the Transact SQL query, and the SQL Server Management Studio.

Create SQL Foreign Key using T-SQL

In this example we will show you, How to create a Foreign Key in SQL server using Transact SQL statement. Here we will create a Foreign key that will connect with the tblDepartment at the creation of a table. I Suggest you to refer Create Table article to understand the Create Table statement.

-- Creating SQL Server Foreign Key
USE [SQLTEST]
GO

CREATE TABLE [dbo].[tblEmployee](
	[EmpID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,
	[FirstName] [nvarchar](255) NULL,
	[LastName] [nvarchar](255) NULL,
	[Education] [nvarchar](255) NULL,
	[YearlyIncome] [float] NULL,
	[Sales] [float] NULL,
	[DeptID] [int] NULL,
	CONSTRAINT [FK_tblDepartment_tblEmployee_DeptID] FOREIGN KEY([DeptID])
		REFERENCES [dbo].[tblDepartment] ([ID])
) 
GO

As you can see from the above code snippet, we added the CONSTRAINT statement at the end of the table creation. CONSTRAINT Constraint_Name FOREIGN KEY (Column_Name of tblEmployee) REFERENCES PrimaryKey_Table (Key Column).

OUTPUT

Let me use the SP_HELP procedure to check the table definition, and the constraints

From the above screenshot you can see our newly created foreign key. You can also use the Object Explorer to achieve the same. To do so, Expand the tblEMployee, and expand Keys folder

Create SQL Foreign Key on Existing table

In this example we will show you, How to add Foreign Key on an existing table. For this, we are going to create a new table using the following statement

USE [SQLTEST]
GO

CREATE TABLE [dbo].[tblEmployee](
	[EmpID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,
	[FirstName] [nvarchar](255) NULL,
	[LastName] [nvarchar](255) NULL,
	[Education] [nvarchar](255) NULL,
	[YearlyIncome] [float] NULL,
	[Sales] [float] NULL,
	[DeptID] [int] NULL
) 
GO

OUTPUT

Now let me add the Foreign key constraint. As you can see from the below screenshot we are using the Alter Table Statement to alter the table content, and then used the ADD Constraint statement to add Foreign key constraint.

USE [SQLTEST]
GO
ALTER TABLE [dbo].[tblEmployee]  
ADD CONSTRAINT [FK_tblDepartment_tblEmployee_DeptID] FOREIGN KEY([DeptID])
			REFERENCES [dbo].[tblDepartment] ([ID])

OUTPUT

Create SQL Foreign Key using SSMS

In this example we will show you, How to create a Foreign Key in SQL server using the SQL server management Studio. To do so, Please go to the Object Explorer. Within the object explorer, Expand the Database folder, and select the table on which you want to create a Foreign key. Next, Right click on it will open the context menu. Please select Design option as shown below

Once you click the Design option, SQL management studio will open the corresponding table in design mode. Right click on the DeptID column will open the context menu. Please select the Relationships option from the available options.

As you can see there are no existing relationships. Please click on the Add button to add new relation.

Once you select the Add option, SSMS will create a Foreign key for you.

Next, click on the Browse button beside the Tables and Columns Specification option. This property is used to specify the relation between two tables.

We selected the Primary Key table as the tblDepartment, and Column as ID.

Next, we selected the Foreign key table as tblEmployee, and key column as the Dept ID

Modify SQL Foreign Key

Within the object explorer, Expand the Database folder in which the table exists. Please select the table on which your Foreign key holds, then go to the Keys folder. Next, Right click on the key name will open the context menu. Please select the Modify option

Once you click the Modify option, SQL management studio will open the corresponding table in design mode, along with the Foreign Key Relationships window.

NOTE : You can’t alter the foreign constraint using the SQL query. In order to alter the existing foreign key, you have to drop (or delete) and recreate it.

Delete SQL Foreign Key using T-SQL

Please use the DROP Constraint statement along with the ALTER TABLE Statement to delete or drop Foreign key constraint

USE [SQLTEST]
GO
ALTER TABLE [dbo].[tblEmployee]  
DROP CONSTRAINT [FK_tblDepartment_tblEmployee_DeptID]

OUTPUT 

Delete SQL Foreign Key Constraint using SSMS

Within the object explorer, Expand the Database folder, and select the table on which your Foreign key holds, then go to the Keys folder. Next, Right click on the key name will open the context menu. Please select the Delete option

Once you select the Delete option, Delete Object window will be opened. Click OK to delete the foreign key constraint.

You can see that there is no Foreign Key

Delete SQL Foreign Key Constraint using SSMS 2

Within the object explorer, Expand the Database folder and Right click on the table name, and click the Design option. Next, select the Relationships option from the context menu to open the following window. Now, Please select the Foreign Key, and click on the Delete button.

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