What is DataRelation object in ADO.NET? How to use a DataRelation between two columns in ADO.NET?

In order to set the relationship between two or more than two columns, ADO.NET provides the DataRelation class. When a DataRelation object is created, it assists to enforce some constraints on the relationships between columns. The constraint may be like a Unique constraint that ensures that a column will have no duplicate value in the table. A Foreign Key constraint may be used to enforce Referential Integrity. The Unique property may be set by setting the Unique property of a DataColumn to True. This may also be done by adding an instance of the UniqueConstraint class to the DataRelation object. As a part of the foreign key constraint, we may specify referential integrity rules that are applied at 3 places

1) When a parent record is updated
2) When a parent record is deleted
3) When a change is rejected or accepted.

A DataRelation object permits to establish a parent-child relationship between two or more tables inside a DataSet object. The easiest way to create a DataRelation between two tables in a DataSet is to setup a primary key - foreign key relationship between the columns of a table.

See code example below, where a DataRelation has been setup between the Employee table and the Salary table...

'Code Below in VB.NET
Dim Conn As SqlConnection
Dim da As SqlDataAdapter
Dim ds As DataSet
Dim RowParent As DataRow
Dim RowChild As DataRow

Conn = New _
SqlConnection(ConfigurationSettings.Appsettings("SomeConnectionStringWrittenInWeb.Config"))
da = New SqlDataAdapter("SELECT * FROM Employees", Conn)
ds = New DataSet()

Try
   Conn.Open()
   da.Fill( ds,"Employees")
   da.SelectCommand = New SqlCommand("SELECT * FROM Salary", Conn)
   da.Fill(ds, "Salary")
Catch ex As SqlException
   Response.Write(ex.ToString())
Finally
   Conn.Dispose()
End Try

'Next, Let us create a Data Relationship
ds.Relations.Add("Employee_Salary", ds.Tables("Employees").Columns("EmployeeID"), _
ds.Tables("Salary").Columns("EmployeeID"))
'Display the Employee and Child Salary in the Form
'Say we have a Label in the form
For each RowParent in ds.Tables("Employees").Rows

   lblRelation.Text &= RowParent("Emp_Name")
   For each RowChild in RowParent.GetChildRows("Employee_Salary")
     lblRelation.Text &= "<br/>" & RowChild("Sal_Amount")
   Next
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