Problem to Dropdownlist

Hi,
I have the codes like
 

        sql = LibUtil.GetSqlCodeVG(accessRight, dept);
        dt = DBUtil.GetCodeTable(conn, sql);
        ddlStore.DataSource = dt;
        ddlStore.DataValueField = "code";
        ddlStore.DataTextField = "dscr";
        ddlStore.DataBind();
 
    public static string GetSqlCodeVG(string accessRight, string dept)
    {
        string sql = @"select store_no code, sfzone, concat(ifnull(dscr,''),',  ',ifnull(store_no,'')) as dscr
            from c_store_vg c
            where deleted = '0' and dscr is NOT NULL ";
 
        sql += "order by dscr ";
 
        return sql;
    }
 

Select allOpen in new window

to fill the Dropdownlist. But after I've selected one value to the DDL, I check that
 
ddlStore.SelectedValue
 
is having empty value inside. why?