[Web,C#] มีปัญหาเรื่อง เพิ่ม Select All ใน Dropdownlist แล้ว Search ไม่เจอค่ะ

SqlCommand cmd = new SqlCommand("Select id_item,id_noy,txt,date_st,time_st,date_sp,time_sp,ti_cs,cause,ty,cs,cause,tw,id_cu,dep_df FROM dbo.tbl_blue LEFT JOIN dbo.tbl_yellow ON dbo.tbl_blue.id_nob=dbo.tbl_yellow.id_noy WHERE dbo.tbl_blue.archive_date IS NULL AND dbo.tbl_blue.st = 2 AND [color=yellow]dbo.tbl_blue.id_cu = '" + ddlDep.SelectedItem.Value + "' AND dbo.tbl_yellow.date_st BETWEEN '" + ASPxDtst.Date.ToString("yyyy-MM-dd") + "' AND '" + ASPxDtsp.Date.ToString("yyyy-MM-dd") + "' GROUP BY id_item,id_noy,txt,date_st,time_st,date_sp,time_sp,ti_cs,cause,ty,cs,cause,tw,id_cu,dep_df", con);

            StringBuilder query = new StringBuilder();
            query.Append(" Select id_item,id_noy,txt,date_st,time_st");
            query.Append(" ,date_sp,time_sp,ti_cs,cause,ty,cs,cause,tw,id_cu,dep_df ");
            query.Append(" FROM dbo.tbl_blue LEFT JOIN dbo.tbl_yellow");
            query.Append(" ON dbo.tbl_blue.id_nob=dbo.tbl_yellow.id_noy");
            query.Append(" WHERE dbo.tbl_blue.archive_date IS NULL");
            query.Append(" AND dbo.tbl_blue.st = 2 ");
            //ถ้าเลือกไม่เลือก ALL ให้ค้นหาตาม dbo.tbl_blue.id_cu 
            if (int.Parse(ddlDep.SelectedItem.Value.Tostring()) != 0)
            {
                query.Append(" AND dbo.tbl_blue.id_cu = '" + ddlDep.SelectedItem.Value.Tostring() + "'");
            }
           
            query.Append(" AND dbo.tbl_yellow.date_st BETWEEN '" + ASPxDtst.Date.ToString("yyyy-MM-dd") +"'");
            query.Append(" AND '" + ASPxDtsp.Date.ToString("yyyy-MM-dd") + "' ");
            query.Append(" GROUP BY id_item,id_noy,txt,date_st,time_st,date_sp");
            query.Append(" ,time_sp,ti_cs,cause,ty,cs,cause,tw,id_cu,dep_df");

            SqlCommand cmd = new SqlCommand(query.ToString(), con);
            SqlDataReader dr = cmd.ExecuteReader();

protected void BindContrydropdown()
        {
            using (SqlConnection con = new SqlConnection("Server=server06;Uid=mdsadmin;PASSWORD=PsK954367xZ;database=MDS;Max Pool Size=400;Connect Timeout=600;"))
        {
            con.Open();

            string sqlStatement = "SELECT id_itemtype + ' / ' + itemtype AS type, id_itemtype FROM dbo.tbl_itemtype";            
            SqlCommand cmd = new SqlCommand(sqlStatement, con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
                
            ddlDep.DataSource = ds;
            ddlDep.DataTextField = "type";
            ddlDep.DataValueField = "id_itemtype";
            ddlDep.DataBind();
            ddlDep.Items.Insert(0, new ListItem("All", "0"));            
            con.Close();
        }
    }

if (ddlDep.SelectedIndex != 0)
                {
                    query.Append(" AND dbo.tbl_blue.id_cu = '" + ddlDep.SelectedItem.Value.ToString() + "'");
                }