procedure SetSource(var aSrc : TDataSource);
begin
  aSrc.DataSet.Close;
  dbgridShowData.DataSource:=aSrc;
  aSrc.DataSet.Open;
  aSrc.DataSet.First;
  aSrc.DataSet.Refresh;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DBGrid1.DataSource := DataSource1;

  DataSet1.Active := true;
  DataSet2.Active := true;
  DataSet3.Active := true;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  DataSource1.DataSet := DataSet1;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  DataSource1.DataSet := DataSet2;
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
  DataSource1.DataSet := DataSet3;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DataSource1.DataSet := DataSet1;
  DataSource2.DataSet := DataSet2;
  DataSource3.DataSet := DataSet3;

  DataSet1.Active := true;
  DataSet2.Active := true;
  DataSet3.Active := true;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  DBGrid1.DataSource := DataSource1;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  DBGrid1.DataSource := DataSource2;
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
  DBGrid1.DataSource := DataSource3;
end;

procedure SetDataFromDataSet(const aDataSource: TDataSource;
  const aNewDataSet: TDataSet);
begin
  aDataSource.DataSet.Close;
  aDataSource.DataSet := aNewDataSet;
  if not aNewDataSet.Active then
    aNewDataSet.Open;
end;

SetDataFromDataSet(DataSource1, CustomerQuery);

procedure TForm1.setDataSourceDataSet(var newDataSource:TDataSource);
begin
if DBgrid1.DataSource = nil then begin
   DBgrid1.DataSource:=newDataSource;
end else begin
if DBgrid1.DataSource.Name = newDataSource.Name then exit;
DBGrid1.DataSource.Enabled:=False;
DBgrid1.DataSource:=newDataSource;
end;
If DBgrid1.DataSource.DataSet.active=False then DBgrid1.DataSource.DataSet.active:=True;
DBGrid1.DataSource.Enabled:=True;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
setDataSourceDataSet(DataSource1);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
setDataSourceDataSet(DataSource2);
end;