如何在 SQL 中从 my_table WHERE some_other_column = "something" 中选择 MAX(column) 作为 max_value

Question

下面是我的 SQL 表。我正在尝试获取time 的最大值，其中to_address 是dfhuih。

我试过了：

SELECT MAX(time) as max_time FROM transactions WHERE to_address='dfhuih'

但是，无论to_address 是什么，该查询的结果都给出了表中的最大time 值——使用PHP mysqli 时。

在指定to_address 的time 列中选择最大值的正确方法是什么？

transactions
 ---------------------------------------
| to_address |     time     |  amount  |
----------------------------------------
|  dfhuih    | 1536294884   |  0.320   |
|  dfhuih    | 1536343222   |  0.564   |
|  adslsd    | 1546294884   |  0.320   |
|  vshjfof   | 1536294884   |  0.320   |
----------------------------------------

我的应用程序中的完整代码是

$stmt1 = $db_connection->prepare("SELECT MAX(time) as max_time FROM transactions WHERE to_address=? AND from_address=? AND is_affiliate_payment=0");
$stmt1->bind_param('is', $user_id, $faucet_key);
$stmt1->execute();
$result1 =$stmt1->get_result();

while($row = $result1->fetch_assoc())
{
   echo $row['max_time']    
}

Answer 1

问题出在这两行：

$stmt1 = $db_connection->prepare("SELECT MAX(time) as max_time 
                                  FROM transactions 
                                  WHERE to_address = ? AND 
                                        from_address = ? AND 
                                        is_affiliate_payment = 0");
$stmt1->bind_param('is', $user_id, $faucet_key);

您的查询中有两个?。第一个是to_address，第二个是from_address。两者都是字符串。然而，在您的bind_param() 中，您要求一个整数to_address。所以代码应该是：

$stmt1 = $db_connection->prepare("SELECT MAX(time) as max_time 
                                  FROM transactions 
                                  WHERE to_address = ? AND 
                                        from_address = ? AND 
                                        is_affiliate_payment = 0");
$stmt1->bind_param('ss', $to_address, $from_address);

请注意，我还更改了其余参数。不知何故$user_id, $faucet_key 似乎不正确。检查一下！

Answer 2

我相信您正在寻找，但很难说没有示例数据和预期结果的使用。

SELECT 
 transactions.*
FROM (
 SELECT 
   MAX(time) AS max_time
 FROM 
  transactions
 WHERE
   to_address = 'dfhuih'
) AS transactions_filter
INNER JOIN 
 transactions  
ON
   transactions_filter.max_time = transactions.time   
 AND  
   transactions.to_address = 'dfhuih'

请记住，如果两个 MAX 时间值相同，它也会显示平局

在 MySQL 8 中，您还可以将该查询重写为下面的查询。
请注意，列别名作为列名移动到派生表别名。

SELECT 
 transactions.*
FROM (
 SELECT 
   MAX(time)
 FROM 
  transactions
 WHERE
   to_address = 'dfhuih'
) AS transactions_filter(max_time) # <- here is the column alias also defined.
INNER JOIN 
 transactions  
ON
   transactions_filter.max_time = transactions.time   
 AND  
   transactions.to_address = 'dfhuih'