create cascading dropdownlist from cached data

Question

In my application I have written a main query like

 SqlDataAdapter cmd0 = new SqlDataAdapter(select a,b,c,d, from table1,sqlconn)

then I have cached this query since table1 is a very huge table and I don't want to call the database everytime user makes some change.

        DataSet ds0 = new DataSet();
        cmd0.Fill(ds0);
        DataView source0 = new DataView(ds0.Tables[0]);
        DataTable dt0 = new DataTable();
        cmd0.Fill(dt0);
        Cache["data"] = ds0;

I have 2 dropdownlists for a and b. I get the cached column a and bind the data in the dropdownlists

 //dropdown for a
        DataSet dataset_a = new DataSet();
        dataset_a = (DataSet)Cache["data"];
        DataView dataview_a = dataset_a.Tables[0].DefaultView;
        dataview_a.Sort = "a";
        DataTable datatable_a = dataview_a.ToTable(true, "a");
        ddla.DataSource = datatable_a;
        ddla.DataTextField = "a";
        ddla.DataValueField = "a";
        ddla.DataBind();

Now I try to populate dropdownlist for b initally by the same way as I did for ddla

//dropdown for b
        DataSet dataset_b = new DataSet();
        dataset_b = (DataSet)Cache["data"];
        DataView dataview_b = dataset_b.Tables[0].DefaultView;
        dataview_b.Sort = "b";
        DataTable datatable_b = dataview_b.ToTable(true, "b");
        ddlb.DataSource = datatable_b;
        ddlb.DataTextField = "b";
        ddlb.DataValueField = "b";
        ddlb.DataBind();

when I change my selection in ddla(dropdownlist for a) I want the dropdown list for b also to be changed. (Cascading dropdownlists)

So basically I want to fire the query like:

Select b from table1 where a=ddla.SelectedItem.ToString()

but I don't kno how to do it with the cached data.

Please help me! Thank you!!!

Answer 1

You can't bind to same source and expect to get result you looking for. To get what you want you need to unbind ddlb. And instead, on event SelectedIndexChanged or SelectedItemChanged of ddla you should filter the rows from the data table and load into the ddlb. There are actually few ways of doing it. One, is to have 2 tables in dataset and use relationship, with one combo bind to table a and another bind to table b. This way, when you click in a, the related rows will show up in b