我需要在另一个中使用一个mySQL查询的输出（它的值）

问题描述：

I have trouble using the output of one query as a value in another. The thing is I need to get the price of one product (which is successfully accomplished) using the following query:

SELECT Cena FROM Proizvod WHERE ProzivodID = $PID

$PID is a PHP variable and this query works just fine. The problem comes when I want to subtract the price of the product from the buyer's balance. I tried to do that with the following query:

UPDATE Kupuvac SET Saldo = Saldo - ".$cena." WHERE KupuvacID=$KID

$cena is a PHP variable, the value of which is the first query.

The second query doesn't work at all, it doesn't return an error message, but it doesn't change he buyer's balance.

A short dictionary if someone needs it: Cena - Price; Proizvod - Product; Kupuvac - Buyer; Saldo - Balance.

Thanks in advance!

我无法将一个查询的输出用作另一个查询的值。问题是我需要使用以下查询来获得一个产品（成功完成）的价格： p>

  SELECT Cena FROM  Proizvod WHERE ProzivodID = $ PID 
  code>  pre> 
 
  $ PID是一个PHP变量，这个查询工作正常。 当我想从买方的余额中减去产品的价格时出现问题。
我尝试使用以下查询来做到这一点： p> 
 
 
   UPDATE Kupuvac SET Saldo = Saldo  - “。$ cena。” 在哪里KupuvacID = $ KID 
  code>  pre> 
 
  $ cena是一个PHP变量，其值是第一个查询。 p> 
 
 
 第二个查询根本不起作用，它不会返回错误消息，但它不会改变买方的余额。 p> 
 
 

  A 短语如果有人需要它： Cena  -  Price  code>;   Proizvod  -  Product  code>;   Kupuvac  - 买方 code>;   Saldo  -  Balance  code>。 em>  p> 
 
 
提前致谢！ p> 
  div>

答

You can do your UPDATE operation in one statement:

UPDATE Kupuvac a
CROSS JOIN
(
    SELECT Cena FROM Proizvod WHERE ProzivodID = $PID
) b
SET a.Saldo = a.Saldo - b.Cena
WHERE a.KupuvacID = $KID

答

Can't you just combine these into one query?

UPDATE Kupuvac 
SET Saldo = Saldo - (SELECT Cena FROM Proizvod WHERE ProzivodID = $PID) 
WHERE KupuvacID = $KID

我需要在另一个中使用一个mySQL查询的输出（它的值）

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