php中mysql数据库中两个不相关表的两列之和

Question

我有两个带有金额列的不相关表。 我想将两列的金额相加并减去两个总和，所有这些都是在我正在研究的 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

我想合计请求的金额并将其从购买表中的价格总和中减去。

查询不起作用。

Answer 1

例如

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