SQL Server: Foreign Keys

In this post explains how to use Foreign Keys in SQL Server with syntax and examples.

What is a foreign key in SQL Server?

A foreign key is a way to enforce referential integrity within your SQL Server database. A foreign key means that values in one table must also appear in another table.

The referenced table is called the parent table while the table with the foreign key is called the child table. The foreign key in the child table will generally reference a primary key in the parent table.

A foreign key can be created using either a CREATE TABLE statement or an ALTER TABLE statement.

Create Foreign key - Using CREATE TABLE statement

Syntax

The syntax for creating a foreign key using a CREATE TABLE statement in SQL Server (Transact-SQL) is:

CREATE TABLE child_table
(
  column1 datatype [ NULL | NOT NULL ],
  column2 datatype [ NULL | NOT NULL ],
  ...

  CONSTRAINT fk_name
    FOREIGN KEY (child_col1, child_col2, ... child_col_n)
    REFERENCES parent_table (parent_col1, parent_col2, ... parent_col_n)
    [ ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
    [ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ] 
);

child_table

The name of the child table that you wish to create.

column1, column2

The columns that you wish to create in the table. Each column must have a datatype. The column should either be defined as NULL or NOT NULL and if this value is left blank, the database assumes NULL as the default.

fk_name

The name of the foreign key constraint that you wish to create.

child_col1, child_col2, ... child_col_n

The columns in child_table that will reference a primary key in the parent_table.

parent_table

The name of the parent table whose primary key will be used in the child_table.

parent_col1, parent_col2, ... parent_col3

The columns that make up the primary key in the parent_table. The foreign key will enforce a link between this data and the child_col1, child_col2, ... child_col_n columns in the child_table.

ON DELETE

Optional. It specifies what to do with the child data when the parent data is deleted. You have the options of NO ACTION, CASCADE, SET NULL, or SET DEFAULT.

ON UPDATE

Optional. It specifies what to do with the child data when the parent data is updated. You have the options of NO ACTION, CASCADE, SET NULL, or SET DEFAULT.

NO ACTION

It is used in conjunction with ON DELETE or ON UPDATE. It means that no action is performed with the child data when the parent data is deleted or updated.

CASCADE

It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is either deleted or updated when the parent data is deleted or updated.

SET NULL

It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is set to NULL when the parent data is deleted or updated.

SET DEFAULT

It is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is set to their default values when the parent data is deleted or updated.

Example

Let's look at an example of how to create a foreign key in SQL Server (Transact-SQL) using the CREATE TABLE statement.

For example:

CREATE TABLE products
( product_id INT PRIMARY KEY,
  product_name VARCHAR(50) NOT NULL,
  category VARCHAR(25)
);

CREATE TABLE inventory
( inventory_id INT PRIMARY KEY,
  product_id INT NOT NULL,
  quantity INT,
  min_level INT,
  max_level INT,
  CONSTRAINT fk_inv_product_id
    FOREIGN KEY (product_id)
    REFERENCES products (product_id)
);

In this foreign key example, we've created our parent table as the products table. The products table has a primary key that consists of the product_id field.

Next, we've created a second table called inventory that will be the child table in this foreign key example. We have used the CREATE TABLE statement to create a foreign key on the inventory table called fk_inv_product_id. The foreign key establishes a relationship between the product_id column in the inventory table and the product_id column in the products table.

This first example shows how to create a foreign key involving one column. Let's look at how to create a foreign key with one than one field.

For example:

CREATE TABLE products
( product_name VARCHAR(50) NOT NULL,
  location VARCHAR(50) NOT NULL,
  category VARCHAR(25)
  CONSTRAINT products_pk PRIMARY KEY (product_name, location)
);

CREATE TABLE inventory
( inventory_id INT PRIMARY KEY,
  product_name VARCHAR(50) NOT NULL,
  location VARCHAR(50) NOT NULL,
  quantity INT,
  min_level INT,
  max_level INT,
  CONSTRAINT fk_inv_product
    FOREIGN KEY (product_name, location)
    REFERENCES products (product_name, location)
);

In this foreign key example, our parent table called products has a primary key that consists of both the product_name and location columns. Therefore, our child table and foreign key must also reference these two columns.

So in this example, our foreign key called fk_inv_product references the products table based on two fields - the product_name and location fields.

Create a foreign key - Using ALTER TABLE statement

Syntax

The syntax for creating a foreign key using an ALTER TABLE statement in SQL Server (Transact-SQL) is:

ALTER TABLE child_table
ADD CONSTRAINT fk_name
    FOREIGN KEY (child_col1, child_col2, ... child_col_n)
    REFERENCES parent_table (parent_col1, parent_col2, ... parent_col_n);

child_table

The name of the child table that you wish to modify.

fk_name

The name of the foreign key constraint that you wish to create.

child_col1, child_col2, ... child_col_n

The columns in child_table that will reference a primary key in the parent_table.

parent_table

The name of the parent table whose primary key will be used in the child_table.

parent_col1, parent_col2, ... parent_col3

The columns that make up the primary key in the parent_table. The foreign key will enforce a link between this data and the child_col1, child_col2, ... child_col_n columns in the child_table.

Example

Let's look at an example of how to create a foreign key in SQL Server (Transact-SQL) using the ALTER TABLE statement.

For example:

ALTER TABLE inventory
ADD CONSTRAINT fk_inv_product_id
    FOREIGN KEY (product_id)
    REFERENCES products (product_id);

In this foreign key example, we've created a foreign key on the inventory table called fk_inv_product_id that references the products table based on the product_id field.

We could also create a foreign key with more than one field as in the example below:

ALTER TABLE inventory
ADD CONSTRAINT fk_inv_product
    FOREIGN KEY (product_name, location)
    REFERENCES products (product_name, location);

In this SQL Server example, we have created a foreign key on the inventory table called fk_inv_product that references the products table based on the product_name and location columns.