Pre select listbox with sql column values

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C#

ASP.NET

SQL

my listbox has severals values. I am trying to pre select my listbox at page load but the problem is it only select the first value.
Suppose i have values in my listbox from 1-10 and in database column i have 1,4,7 then it pre select only 1 in the listbox.
If i have 1,2,7 then it preselect 1 and 2
If i have 1,2,3 then it preselect 1,2 and 3
So it pre select only together values if any gap occur then it select upper value.

<asp:ListBox ID="dd_subcategory" runat="server" CssClass="lbcss" SelectionMode="Multiple">
</asp:ListBox>

private void FillSubCategory(int CategoryID)
    {
        SqlCommand cmd = new SqlCommand("sps_bindcategory", con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@flag", 2);
        cmd.Parameters.AddWithValue("@CategoryID", CategoryID);
        DataSet objDs = new DataSet();
        SqlDataAdapter dAdapter = new SqlDataAdapter();
        dAdapter.SelectCommand = cmd;
        con.Open();
        dAdapter.Fill(objDs);
        con.Close();
        if (objDs.Tables[0].Rows.Count > 0)
        {
            dd_subcategory.DataSource = objDs.Tables[0];
            dd_subcategory.DataTextField = "subcategory_name";
            dd_subcategory.DataValueField = "subcategory_id";
            dd_subcategory.DataBind();            
        }
    }

if (!Page.IsPostBack)
        {
            DataTable dt = GetData();
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    string id = dt.Rows[i]["subcategory_id"].ToString();
                    if (id == dd_subcategory.Items[i].Value)
                    {
                        dd_subcategory.Items[i].Selected = true;
                    }
                }
            }
        }
private DataTable GetData()
    {

        DataTable dt = new DataTable();
        try
        {
            con.Open();
            SqlCommand cmd = new SqlCommand("sps_addetails", con);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@ad_id", ad_id);
            cmd.Parameters.AddWithValue("@useremail", ses);
            SqlDataAdapter DA = new SqlDataAdapter(cmd);
            DA.Fill(dt);
            con.Close();
        }
        catch (SqlException)
        {
            throw;
        }
        return dt;
    }

ALTER PROCEDURE [dbo].[sps_addetails] 
	@ad_id int,
	@useremail nvarchar(100)
AS
BEGIN
	select r.*, sc.subcategory_id from dbo.tbl_adregister r INNER JOIN tbl_adsubcategory sc on r.ad_id=sc.ad_id
	where useremail=@useremail and r.ad_id=@ad_id
END