Invalid Object Name Error While Constructing Sql Using Datarelation

 Hi mates
I am getting the following error "Invalid object name 't1'.Invalid object name 'relT1T2'."
 from this line of code "  String SQLREL = "SELECT t1.SubCategory2Name, t1.CityName, SubCategory2Name,relT1T2.N FROM " +
       "t1 INNER JOIN relT1T2 ON t1.PKSUBCAT2ID = relT1T2.FKSUBCAT2ID";"
i am basically using datarelation and the relation is working fine and is showing all the data for the parent and child tables but while constructing sql i get above mentioned error.can some one pls help me out. 
the full code is given below:         DataColumn parentCol;
            DataColumn childCol;

            parentCol = ds11.Tables["t1"].Columns["PKSUBCAT2ID"];
            childCol = ds11.Tables["t2"].Columns["FKSUBCAT2ID"];





DataRelation relT1T2;
            relT1T2 = new DataRelation("T1T2", parentCol, childCol);

            ds11.Relations.Add(relT1T2);

 


            Response.Write(ds11.Tables["t1"].Rows.Count);
     


            foreach (DataRow rowParent in ds11.Tables["t1"].Rows)
            {
                foreach (DataRow rowChild in rowParent.GetChildRows(relT1T2))
                {
                    Response.Write(ds11.Tables["t1"].Rows.Count);//test code
                    DataView dv1 = ds11.Tables["t1"].DefaultView;
                 



      String SQLREL = "SELECT t1.SubCategory2Name, t1.CityName, SubCategory2Name,relT1T2.N FROM " +
       "t1 INNER JOIN relT1T2 ON t1.PKSUBCAT2ID = relT1T2.FKSUBCAT2ID";

                    DataSet ds77 = new DataSet();
                    SqlConnection cn77 = new SqlConnection(CONN1);
                    cn77.Open();
                    SqlDataAdapter da77 = new SqlDataAdapter(SQLREL, cn77);
                    da77.Fill(ds77);

                    DataList1.DataSource = ds77.Tables[0].DefaultView;

                    DataList1.DataBind();
                    cn77.Close();