else if (combo_View.Text == "Orders")
{

    da.SelectCommand = new OleDbCommand("SELECT * FROM TestQuery WHERE (VendorName = @VendorName OR @VendorName = '') AND (CustomerName = @CustomerName OR @CustomerName = '') AND (PO = @PO OR @PO = '')  AND (ItemNum = @ItemNum OR @ItemNum = '') AND (orderDate BETWEEN @From AND @To) ORDER BY CustomerName", cs);

    //da.SelectCommand = new OleDbCommand("SELECT * FROM TestQuery WHERE orderDate BETWEEN @From AND @To", cs);

    da.SelectCommand.Parameters.Add("@VendorName", OleDbType.VarChar).Value = combo_VendorView.Text.ToString();
    da.SelectCommand.Parameters.Add("@CustomerName", OleDbType.VarChar).Value = combo_CustomerView.Text.ToString();

    da.SelectCommand.Parameters.Add("@From", OleDbType.Date).Value = "#" + tp_viewFrom.Value.Date.ToString("M/d/yyyy") + "#";
    da.SelectCommand.Parameters.Add("@To", OleDbType.Date).Value = "#" + tp_viewTo.Value.Date.ToString("M/d/yyyy") + "#";

    da.SelectCommand.Parameters.Add("@PO", OleDbType.VarChar).Value = txt_POLookup.Text.ToString();
    da.SelectCommand.Parameters.Add("@ItemNum", OleDbType.VarChar).Value = combo_ItemNumLookup.Text.ToString();


    dsB.Clear();
    da.Fill(dsB);
    dgv_DataLookup.DataSource = dsB.Tables[0];
}
	
da.SelectCommand = new OleDbCommand("SELECT * FROM TestQuery WHERE
    (VendorName = @VendorName OR @VendorName = '')
    AND (CustomerName = @CustomerName OR @CustomerName = '')
    AND orderDate BETWEEN @From AND @To ORDER BY CustomerName", cs);
	
and (@from is null or @to is null or <between stmt> )
	
VendorName = @VendorName OR @VendorName = ''