Sorting gridview Each Coulmn Using Sorting Technique

protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
{
ViewState["a"] = e.SortExpression;
GridView1.DataSource = bindgrid();
GridView1.DataBind();
}
DataView dv;
private DataView bindgrid()

{
SqlDataAdapter da1 = new SqlDataAdapter("Select * From enquiry", sqlCon);
DataSet ds1 = new DataSet();
da1.Fill(ds1);
if (ViewState["a"] != null)
{
dv = new DataView (ds1.Tables[0]);
dv.Sort = (string)ViewState["a"];
}
else
{
dv = ds.Tables[0].DefaultView; ;
}
return dv;
}