A foreign key constraint doesn't have to be linked only to a primary key constraint in another table. Foreign keys can also be defined to reference the columns of a UNIQUE constraint in another table.

When a value other than NULL is entered into the column of a FOREIGN KEY constraint, the value must exist in the referenced column. Otherwise, a foreign key violation error message is returned. To make sure that all values of a composite foreign key constraint are verified, specify NOT NULL on all the participating columns.

FOREIGN KEY constraints can reference only tables within the same database on the same server. Cross-database referential integrity must be implemented through triggers. For more information, see CREATE TRIGGER .

FOREIGN KEY constraints can reference another column in the same table, and is referred to as a self-reference.

A FOREIGN KEY constraint specified at the column level can list only one reference column. This column must have the same data type as the column on which the constraint is defined.

A FOREIGN KEY constraint specified at the table level must have the same number of reference columns as the number of columns in the constraint column list. The data type of each reference column must also be the same as the corresponding column in the column list.

The Database Engine doesn't have a predefined limit on the number of FOREIGN KEY constraints a table can contain that reference other tables. The Database Engine also doesn't limit the number of FOREIGN KEY constraints owned by other tables that reference a specific table. However, the actual number of FOREIGN KEY constraints used is limited by the hardware configuration, and by the design of the database and application. A table can reference a maximum of 253 other tables and columns as foreign keys (outgoing references). SQL Server 2016 (13.x) and later increases the limit for the number of other tables and columns that can reference columns in a single table (incoming references), from 253 to 10,000. (Requires at least 130 compatibility level.) The increase has the following restrictions:

FOREIGN KEY constraints aren't enforced on temporary tables.

If a foreign key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering. For more information, see CLR User-Defined Types .

A column of type varchar(max) can participate in a FOREIGN KEY constraint only if the primary key it references is also defined as type varchar(max) .

Create a foreign key relationship in Table Designer

Use SQL Server Management Studio

In Object Explorer, right-click the table that will be on the foreign-key side of the relationship and select Design .

The table opens in Table Designer .

From the Table Designer menu, select Relationships . (See the Table Designer menu in the header, or, right-click in the empty space of the table definition, then select Relationships... .)

In the Foreign-key Relationships dialog box, select Add .

The relationship appears in the Selected Relationship list with a system-provided name in the format FK_< tablename >_< tablename >, where the first tablename is the name of the foreign key table, and the second tablename is the name of the primary key table. This is simply a default and common naming convention for the (Name) field of the foreign key object.

Select the relationship in the Selected Relationship list.

Select Tables and Columns Specification in the grid to the right and select the ellipses ( ... ) to the right of the property.

In the Tables and Columns dialog box, in the Primary Key drop-down list, choose the table that will be on the primary-key side of the relationship.

In the grid beneath, choose the columns contributing to the table's primary key. In the adjacent grid cell to the right of each column, choose the corresponding foreign-key column of the foreign-key table.

Table Designer suggests a name for the relationship. To change this name, edit the contents of the Relationship Name text box.

Choose OK to create the relationship.

Close the table designer window and Save your changes for the foreign key relationship change to take effect.

Create a foreign key in a new table

Use Transact-SQL

The following example creates a table and defines a foreign key constraint on the column TempID that references the column SalesReasonID in the Sales.SalesReason table in the AdventureWorks database. The ON DELETE CASCADE and ON UPDATE CASCADE clauses are used to ensure that changes made to Sales.SalesReason table are automatically propagated to the Sales.TempSalesReason table.

CREATE TABLE Sales.TempSalesReason 
      TempID int NOT NULL, Name nvarchar(50)
      , CONSTRAINT PK_TempSales PRIMARY KEY NONCLUSTERED (TempID)
      , CONSTRAINT FK_TempSales_SalesReason FOREIGN KEY (TempID)
        REFERENCES Sales.SalesReason (SalesReasonID)
        ON DELETE CASCADE
        ON UPDATE CASCADE
Create a foreign key in an existing table
Use Transact-SQL
The following example creates a foreign key on the column TempID and references the column SalesReasonID in the Sales.SalesReason table in the AdventureWorks database.
ALTER TABLE Sales.TempSalesReason
   ADD CONSTRAINT FK_TempSales_SalesReason FOREIGN KEY (TempID)
      REFERENCES Sales.SalesReason (SalesReasonID)
      ON DELETE CASCADE
      ON UPDATE CASCADE
Next steps
Primary and Foreign Key Constraints
GRANT Database Permissions
ALTER TABLE
CREATE TABLE
ALTER TABLE table_constraint.