用php减去mysql数据库中两个不相关表的两列之和

我有两个不相关的表，其中有一个amount列。我想求两列的总和，然后减去两个总和，所有这些都是在我正在开发的API中完成的

  case 'amountleft':
  
 $warehouse = $_POST['warehouse'];
 
 $stmt = $conn->prepare("SELECT SUM(price) AS amount FROM purchases WHERE warehouse = ?");
 $stmttwo = $conn->prepare("SELECT SUM(amountrequested) AS amounttwo FROM fundruequest WHERE warehouse = ?");
 
 $stmt->bind_param("s",$warehouse);
 $stmttwo->bind_param("s",$warehouse);
 
 $stmt->execute();
  $stmttwo->execute();
 
 $stmt->store_result();
 $stmttwo->store_result();
 
 if($stmt->num_rows > 0 && $stmttwo->num_rows > 0){
 

 $stmt->bind_result($amount);
 $stmttwo->bind_result($amounttwo);
 $stmt->fetch();
 $stmttwo->fetch();

 $newamount = $stmttwo - $stmt;
 
 $newamount = array(
 'newamount'=>$newamount
 );
 
 $response['error'] = false; 
 $response['message'] = 'Amount Allocated Gotten'; 
 $response['newamount'] = $newamount; 
 }else{
 $response['error'] = false; 
 $response['message'] = 'Amount Allocated Not GOtten';
 }
 
 
 break;

表结构

purchases table.
ID               Price
1                 2
2                 34
4                 12
5                 87

fundruequest Table.
ID               amountrequested
1                 89
2                 67
4                 32
5                 56

我想计算所需金额的总和，并从采购表中的价格总和中减去它

查询不起作用。

例如

SELECT SUM(price) AS amount
  FROM 
(
SELECT price, warehouse FROM purchases
UNION ALL
SELECT price * -1, warehouse FROM sales
) x
WHERE warehouse = ?;

---

您可以尝试从数据库中获取值，并在phphow中管理减法/加法。请，我尝试过了，但它不起作用。您可以共享您的表结构吗？@AnkitJindal我尝试了您所说的，我编辑了问题以反映这一点，但它仍然不起作用，它看起来比编写SQL查询更简单，SQL查询可以为您进行求和和和相减。有关更多帮助，请参阅：抱歉，它工作得非常好。多谢各位