$col = array();
    $col["title"] = "Total ordered";
    $col["name"] = "total_order";
    $col["search"] = true;
    $col["editable"] = false;
    $cols[] = $col;

$g->select_command = "SELECT m.material_id, SUM(orders.material_quantity) as total_order
                      FROM materials m
		      LEFT JOIN orders
		      ON m.material_id = orders.material_id
		      GROUP BY m.material_id
		      ";