problem with fetching data.

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ASP.NET

I have one list box when i will select item from list box that time all subitems should be disply from database i have tried following code its working only for some items,its not working for all items.plz help me?

protected void lbcategory_SelectedIndexChanged(object sender, EventArgs e)
    {
        Panelconstruction.Visible = false;
        SqlConnection sqlCon = new SqlConnection();
        sqlCon.ConnectionString = "Data Source=PC-13\\SQLEXPRESS;Initial Catalog=DbPlacementCoordinator;Integrated Security=True";
        sqlCon.Open();
        string str = lbcategory.SelectedItem.Value;
        string query = "Select Industry_ID from Industry_Category where Industry_Name=@str;";
        SqlCommand cmd3 = new SqlCommand(query, sqlCon);
        cmd3.Parameters.AddWithValue("str", str);
        int b = Convert.ToInt16(cmd3.ExecuteScalar());
        string jobcat = "Select Job_Name from Job_Category where Industry_ID=@b;";
        SqlCommand cmd4 = new SqlCommand(jobcat, sqlCon);
        cmd4.Parameters.AddWithValue("b", b);
        SqlDataReader reader = cmd4.ExecuteReader();
        inbox.DataSource = reader;
        inbox.DataBind();

        Panelconstruction.Visible = true;

       }