Creating, Altering, and Removing Foreign Keys
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Azure Synapse Analytics
In SQL Server Management Objects (SMO), foreign keys are represented by the ForeignKey object.
To create a foreign key in SMO, you must specify the table on which the foreign key is defined in the constructor of the ForeignKey object. From the table, you must select at least one column to be the foreign key. To do this, create a ForeignKeyColumn object variable and specify the name of the column that is the foreign key. Then, specify the referenced table and referenced column. Use the Add method to add the column to the Columns object property.
The columns that represent the foreign key are listed in the Columns object property of the ForeignKey object. The primary key that is referenced by the foreign key is represented by the ReferencedKey property that is in the table specified in the ReferencedTable property.
Example
To use any code example that is provided, you will have to choose the programming environment, the programming template, and the programming language in which to create your application. For more information, see Create a Visual C# SMO Project in Visual Studio .NET.
Creating, Altering, and Removing a Foreign Key in Visual Basic
This code example shows how to create a foreign key relationship between one or more columns in one table to a primary key column in another table.
'Connect to the local, default instance of SQL Server.
Dim srv As Server
srv = New Server
'Reference the AdventureWorks2022 database.
Dim db As Database
db = srv.Databases("AdventureWorks2022")
'Declare a Table object variable and reference the Employee table.
Dim tbe As Table
tbe = db.Tables("Employee", "HumanResources")
'Declare another Table object variable and reference the EmployeeDepartmentHistory table.
Dim tbea As Table
tbea = db.Tables("EmployeeDepartmentHistory", "HumanResources")
'Define a Foreign Key object variable by supplying the EmployeeDepartmentHistory as the parent table and the foreign key name in the constructor.
Dim fk As ForeignKey
fk = New ForeignKey(tbea, "test_foreignkey")
'Add BusinessEntityID as the foreign key column.
Dim fkc As ForeignKeyColumn
fkc = New ForeignKeyColumn(fk, "BusinessEntityID", "BusinessEntityID")
fk.Columns.Add(fkc)
'Set the referenced table and schema.
fk.ReferencedTable = "Employee"
fk.ReferencedTableSchema = "HumanResources"
'Create the foreign key on the instance of SQL Server.
fk.Create()
Creating, Altering, and Removing a Foreign Key in Visual C#
This code example shows how to create a foreign key relationship between one or more columns in one table to a primary key column in another table.
{
//Connect to the local, default instance of SQL Server.
Server srv;
srv = new Server();
//Reference the AdventureWorks2022 database.
Database db;
db = srv.Databases["AdventureWorks2022"];
//Declare another Table object variable and reference the EmployeeDepartmentHistory table.
Table tbea;
tbea = db.Tables["EmployeeDepartmentHistory", "HumanResources"];
//Define a Foreign Key object variable by supplying the EmployeeDepartmentHistory as the parent table and the foreign key name in the constructor.
ForeignKey fk;
fk = new ForeignKey(tbea, "test_foreignkey");
//Add BusinessEntityID as the foreign key column.
ForeignKeyColumn fkc;
fkc = new ForeignKeyColumn(fk, "BusinessEntityID", "BusinessEntityID");
fk.Columns.Add(fkc);
//Set the referenced table and schema.
fk.ReferencedTable = "Employee";
fk.ReferencedTableSchema = "HumanResources";
//Create the foreign key on the instance of SQL Server.
fk.Create();
}
Creating, Altering, and Removing a Foreign Key in PowerShell
This code example shows how to create a foreign key relationship between one or more columns in one table to a primary key column in another table.
# Set the path context to the local, default instance of SQL Server and to the
#database tables in AdventureWorks2022
CD \sql\localhost\default\databases\AdventureWorks2022\Tables\
#Get reference to the FK table
$tbea = get-item HumanResources.EmployeeDepartmentHistory
# Define a Foreign Key object variable by supplying the EmployeeDepartmentHistory
# as the parent table and the foreign key name in the constructor.
$fk = New-Object -TypeName Microsoft.SqlServer.Management.SMO.ForeignKey `
-argumentlist $tbea, "test_foreignkey"
#Add BusinessEntityID as the foreign key column.
$fkc = New-Object -TypeName Microsoft.SqlServer.Management.SMO.ForeignKeyColumn `
-argumentlist $fk, "BusinessEntityID", "BusinessEntityID"
$fk.Columns.Add($fkc)
#Set the referenced table and schema.
$fk.ReferencedTable = "Employee"
$fk.ReferencedTableSchema = "HumanResources"
#Create the foreign key on the instance of SQL Server.
$fk.Create()
Sample: Foreign Keys, Primary Keys, and Unique Constraint Columns
This sample demonstrates:
Finding a foreign key on an existing object.
How to create a primary key.
How to create a unique constraint column.
The C# version of this sample:
// compile with:
// /r:Microsoft.SqlServer.Smo.dll
// /r:microsoft.sqlserver.management.sdk.sfc.dll
// /r:Microsoft.SqlServer.ConnectionInfo.dll
// /r:Microsoft.SqlServer.SqlEnum.dll
using Microsoft.SqlServer.Management.Smo;
using Microsoft.SqlServer.Management.Sdk.Sfc;
using Microsoft.SqlServer.Management.Common;
using System;
public class A {
public static void Main() {
Server svr = new Server();
Database db = new Database(svr, "TESTDB");
db.Create();
// PK Table
Table tab1 = new Table(db, "Table1");
// Define Columns and add them to the table
Column col1 = new Column(tab1, "Col1", DataType.Int);
col1.Nullable = false;
tab1.Columns.Add(col1);
Column col2 = new Column(tab1, "Col2", DataType.NVarChar(50));
tab1.Columns.Add(col2);
Column col3 = new Column(tab1, "Col3", DataType.DateTime);
tab1.Columns.Add(col3);
// Create the ftable
tab1.Create();
// Define Index object on the table by supplying the Table1 as the parent table and the primary key name in the constructor.
Index pk = new Index(tab1, "Table1_PK");
pk.IndexKeyType = IndexKeyType.DriPrimaryKey;
// Add Col1 as the Index Column
IndexedColumn idxCol1 = new IndexedColumn(pk, "Col1");
pk.IndexedColumns.Add(idxCol1);
// Create the Primary Key
pk.Create();
// Create Unique Index on the table
Index unique = new Index(tab1, "Table1_Unique");
unique.IndexKeyType = IndexKeyType.DriUniqueKey;
// Add Col1 as the Unique Index Column
IndexedColumn idxCol2 = new IndexedColumn(unique, "Col2");
unique.IndexedColumns.Add(idxCol2);
// Create the Unique Index
unique.Create();
// Create Table2
Table tab2 = new Table(db, "Table2");
Column col21 = new Column(tab2, "Col21", DataType.NChar(20));
tab2.Columns.Add(col21);
Column col22 = new Column(tab2, "Col22", DataType.Int);
tab2.Columns.Add(col22);
tab2.Create();
// Define a Foreign Key object variable by supplying the Table2 as the parent table and the foreign key name in the constructor.
ForeignKey fk = new ForeignKey(tab2, "Table2_FK");
// Add Col22 as the foreign key column.
ForeignKeyColumn fkc = new ForeignKeyColumn(fk, "Col22", "Col1");
fk.Columns.Add(fkc);
fk.ReferencedTable = "Table1";
// Create the foreign key on the instance of SQL Server.
fk.Create();
// Get list of Foreign Keys on Table2
foreach (ForeignKey f in tab2.ForeignKeys) {
Console.WriteLine(f.Name + " " + f.ReferencedTable + " " + f.ReferencedKey);
}
// Get list of Foreign Keys referencing table1
foreach (Table tab in db.Tables) {
if (tab == tab1)
continue;
foreach (ForeignKey f in tab.ForeignKeys) {
if (f.ReferencedTable.Equals(tab1.Name))
Console.WriteLine(f.Name + " " + f.Parent.Name);
}
}
}
}
The Visual Basic version of the sample:
' compile with:
' /r:Microsoft.SqlServer.Smo.dll
' /r:microsoft.sqlserver.management.sdk.sfc.dll
' /r:Microsoft.SqlServer.ConnectionInfo.dll
' /r:Microsoft.SqlServer.SqlEnum.dll
Imports Microsoft.SqlServer.Management.Smo
Imports Microsoft.SqlServer.Management.Sdk.Sfc
Imports Microsoft.SqlServer.Management.Common
Public Class A
Public Shared Sub Main()
Dim svr As New Server()
Dim db As New Database(svr, "TESTDB")
db.Create()
' PK Table
Dim tab1 As New Table(db, "Table1")
' Define Columns and add them to the table
Dim col1 As New Column(tab1, "Col1", DataType.Int)
col1.Nullable = False
tab1.Columns.Add(col1)
Dim col2 As New Column(tab1, "Col2", DataType.NVarChar(50))
tab1.Columns.Add(col2)
Dim col3 As New Column(tab1, "Col3", DataType.DateTime)
tab1.Columns.Add(col3)
' Create the ftable
tab1.Create()
' Define Index object on the table by supplying the Table1 as the parent table and the primary key name in the constructor.
Dim pk As New Index(tab1, "Table1_PK")
pk.IndexKeyType = IndexKeyType.DriPrimaryKey
' Add Col1 as the Index Column
Dim idxCol1 As New IndexedColumn(pk, "Col1")
pk.IndexedColumns.Add(idxCol1)
' Create the Primary Key
pk.Create()
' Create Unique Index on the table
Dim unique As New Index(tab1, "Table1_Unique")
unique.IndexKeyType = IndexKeyType.DriUniqueKey
' Add Col1 as the Unique Index Column
Dim idxCol2 As New IndexedColumn(unique, "Col2")
unique.IndexedColumns.Add(idxCol2)
' Create the Unique Index
unique.Create()
' Create Table2
Dim tab2 As New Table(db, "Table2")
Dim col21 As New Column(tab2, "Col21", DataType.NChar(20))
tab2.Columns.Add(col21)
Dim col22 As New Column(tab2, "Col22", DataType.Int)
tab2.Columns.Add(col22)
tab2.Create()
' Define a Foreign Key object variable by supplying the Table2 as the parent table and the foreign key name in the constructor.
Dim fk As New ForeignKey(tab2, "Table2_FK")
' Add Col22 as the foreign key column.
Dim fkc As New ForeignKeyColumn(fk, "Col22", "Col1")
fk.Columns.Add(fkc)
fk.ReferencedTable = "Table1"
' Create the foreign key on the instance of SQL Server.
fk.Create()
' Get list of Foreign Keys on Table2
For Each f As ForeignKey In tab2.ForeignKeys
Console.WriteLine((f.Name + " " + f.ReferencedTable & " ") + f.ReferencedKey)
Next
' Get list of Foreign Keys referencing table1
For Each tab As Table In db.Tables
If (tab.Name.Equals(tab1.Name)) Then
Continue For
End If
For Each f As ForeignKey In tab.ForeignKeys
If f.ReferencedTable.Equals(tab1.Name) Then
Console.WriteLine(f.Name + " " + f.Parent.Name)
End If
Next
Next
End Sub
End Class
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