how to merge Gridview cells to display same value in asp.net

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C#

ASP.NET4

I have a dridview with data source from my database,

my problem is I have a Tutorial but the data is from xml..

My data is from database, how to change it??

THIS TUTORIAL merge data from XML

I WANT TO CREATE A CODE THAT MERGE DATA FROM DATABASE.
AND I DONT KNOW HOW TO DO THAT...

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindGridView();
        }
    }

    protected void gridView_PreRender(object sender, EventArgs e)
    {
        if (rbl.SelectedIndex == 1)
        {
            GridDecorator.MergeRows(gridView);
        }
    }
    protected void rbl_SelectedIndexChanged(object sender, EventArgs e)
    {
        BindGridView();
    }
    void BindGridView()
    {
        DataSet dataSet = new DataSet();
        dataSet.ReadXml(Server.MapPath("~/App_Data/data1.xml"));

        gridView.DataSource = dataSet;
        gridView.DataBind();
    }
}