PHP Mysqli สอบถามเรื่องการ Update ผลการคำนวน 2 ตาราง โดยให้แสดงผลลัพธ์อีกตารางหนึ่ง

 update b
 set b.MaterailDiff = a.qty- a.count
 from mainTable a
 inner join Showtable b on a.id = b.id
where a.desc = 'นม'

<?php
	ini_set('display_errors', 1);
	error_reporting(~0);

	$serverName = "localhost";
	$userName = "root";
	$userPassword = "1234567890";
	$dbName = "projectx";

	$conn = mysqli_connect($serverName,$userName,$userPassword,$dbName);

	$sql = "SELECT * FROM material";

	$query = mysqli_query($conn,$sql);
?>
  <table class="table table-bordered">
  <thead>
    <tr>
    <th scope="col"> เลขวัสดุ</div></th>
    <th scope="col">ชื่อ</div></th>
    <th scope="col">จำนวนคงเหลือ</div></th>
    <th scope="col">จำนวนนับ</div></th>
    <th scope="col">ผลต่าง</div></th>
	  <th scope="col">แก้ไข</div></th>
    </tr>
  </thead>
<?php
while($result=mysqli_fetch_array($query,MYSQLI_ASSOC))
{
?>
  <tr>
    <td><?php echo $result["Material_id"];?></div></td>
    <td><?php echo $result["Material_name"];?></td>
    <td><?php echo $result["Material_qty"];?></td>
    <td><?php echo $result["Material_Count"];?></div></td>
    <td><?php echo $result["Material_diff"];?></td>
	<td><a href="edit.php?Material_id=<?php echo $result["Material_id"];?>">Edit</a></td>
  </tr>
<?php

?>
   <form action="save.php" name="frmAdd" method="post">
  <div class="form-group">
    <label for="Material_id">หมายเลขวัสดุ : </label>
    <input type="hidden" name="Material_id" class="form-control" id="Material_id" value="<?php echo $result["Material_id"];?>"><?php echo $result["Material_id"];?>
  </div>

  <div class="form-group">
    <label for="Material_name">ชื่อวัสดุ : </label>
    <input type="hidden" name="Material_name" class="form-control" id="Material_name" value="<?php echo $result["Material_name"];?>"><?php echo $result["Material_name"];?>
  </div>

  <div class="form-group">
    <label for="Material_qty">จำนวน : </label>
    <input type="hidden" name="Material_qty" class="form-control" id="Material_qty" value="<?php echo $result["Material_qty"];?>"><?php echo $result["Material_qty"];?>
  </div>

  <div class="form-group">
    <label for="Material_Count">จำนวนนับ : </label>
    <input type="text" name="Material_Count" class="form-control" id="Material_Count" value="<?php echo $result["Material_Count"];?>"><?php echo $result["Material_Count"];?>
  </div>
  

  <button type="submit" class="btn btn-primary">Submit</button>
</form>

<?php
	ini_set('display_errors', 1);
	error_reporting(~0);

	$serverName = "localhost";
	$userName = "root";
	$userPassword = "1234567890";
	$dbName = "projectx";

	$conn = mysqli_connect($serverName,$userName,$userPassword,$dbName);


	$sql ="UPDATE material SET Material_diff = '".$_POST["Material_Count"]."'-'".$_POST["Material_qty"]."' Where Material_id='".$_POST["Material_id"]."' ";


	$query = mysqli_query($conn,$sql);

	if($query) {
	 echo "Record update successfully";
	}

	mysqli_close($conn);
?>

$materialId = (int) (isset($_POST["Material_id"]) ? $_POST["Material_id"] : 0);
$materialCount = (int) (isset($_POST["Material_Count"]) ? $_POST["Material_Count"] : 0);

$sql = 'UPDATE `material` SET `Material_Count` = $materialCount, `Material_diff` = `Material_qty` - `Material_Count` WHERE `id` = $materialId'
// เปลี่ยนไปใช้ escape string, prepare (สำหรับ PDO) เอาเองนะ