inner join with DataTables
string ParentKeyColumn = dt1.Columns[0].ColumnName;
 string ChildKeyColumn = dt2.Columns[0].ColumnName;

dt2.PrimaryKey = new DataColumn[] { dt2.Columns["Deptno"] };

DataRelation drel = new DataRelation("EquiJoin", dt2.Columns["Deptno"], dt1.Columns["Deptno"]);

ds.Relations.Add(drel);

var result = new DataTable();

// Create 1 ID column and the non ID columns
var IDColumn = dataTable1.Columns[0];
result.Columns.Add(IDColumn.ColumnName, IDColumn.DataType);
for (var i = 1; i < dataTable1.Columns.Count; i++)
{
  var column = dataTable1.Columns[i];
  result.Columns.Add(column.ColumnName, column.DataType);
}

for (var i = 1; i < dataTable2.Columns.Count; i++)
{
  var column = dataTable2.Columns[i];
  result.Columns.Add(column.ColumnName, column.DataType);
}

// Create the fields container
var fields = new object[result.Columns.Count];

var rows1 = dataTable1.Rows.Cast<DataRow>();
var rows2 = dataTable2.Rows.Cast<DataRow>().ToDictionary(row => (int)row[0]);

// Find the rows which ID is common in table 1 and table 2
foreach (var row1 in rows1)
{
  int ID = (int)row1[0];
  DataRow row2 = null;
  if (rows2.TryGetValue(ID, out row2))
  {
    // Inner join match, add a new row
    int fieldIndex = 0;
    fields[fieldIndex++] = ID;

    for (var i = 1; i < dataTable1.Columns.Count; i++)
    {
      fields[fieldIndex++] = row1[i];
    }

    for (var i = 1; i < dataTable2.Columns.Count; i++)
    {
      fields[fieldIndex++] = row2[i];
    }

    // Add a new row to the result table using the extracted fields from table 1 and table 2
    result.Rows.Add(fields);
  }
}