ผมคิวรี่แบบนี้ไม่ทราบว่าถูกหลักการหรือเปล่าครับ error ไม่แสดง แต่ผลลัพไม่ออกมาครับ echo sql ก็ออกตามต้องการแล้ว

<?php
	$con = mysqli_connect("localhost", "root", "12345", "mng_tree");

	if(mysqli_connect_errno()){
		echo "error connect..?".mysqli_connect_errno($con);
	}else
	{
		echo "Success..? <br><br>";
	}

	$sql0 = "SELECT od.o_id, od.order_id AS o_detal, od.tree_id AS tr_id_od, od.amount AS amn_od, t.tree_id AS tr_id_t, t.tree_name, t.amount AS amn_tree, o.order_id AS oder
			FROM order_detail AS od, tree AS t, order1 AS o
			WHERE od.tree_id = t.tree_id
			AND o.order_id = od.order_id
			AND o.order_id = 20";
	$result = mysqli_query($con, $sql0);
	
	$i = 1;
	while($row = mysqli_fetch_array($result))
	{
		$i++;
		$o_id = $row['o_id'];
		$order_id_OD = $row['o_detal'];
		$order_id_O = $row['oder'];
		$tree_id_OD = $row['tr_id_od'];
		$tree_id_T = $row['tr_id_t'];
		$amount_OD = $row['amn_od'];
		$amount_T = $row['amn_tree'];
		$tree_name = $row['tree_name'];

		echo $o_id." order(".$order_id_O.") order_detal(".$order_id_OD.") tre_id_detal(".$tree_id_OD.") tre_id_tree(".$tree_id_T.") am_detal(".$amount_OD.") am_tree(".$amount_T.") name(".$tree_name.")";
		echo "<br>";
		$delet = $amount_T-$amount_OD;
		echo $delet."<br>";

	if($delet){
		$sql.$i="update tree set amount='$delet' where tree_id = '$tree_id_T' ";
		$dbquery = mysql_db_query("mng_tree", $sql.$i);
	}	
		echo $sql.$i;
		echo "<br>";
	}

mysqli_close($con);


?>