Mysql / PDO-設置變量

Question

我在嘗試設置變量然后在select語句中使用它時遇到了麻煩。 我不斷收到“一般錯誤”，無法弄清楚我在做什么錯。 任何輸入將不勝感激。 我正在嘗試使用帶有命名參數的子查詢來設置變量。

$query = $dbh->prepare("Set @available = (SELECT SUM(payments) FROM payments WHERE customer = :customer) - (SELECT SUM(charges) FROM charges WHERE customer = :customer); SELECT @available");
$query->bindParam(":customer", $customer);
$query->execute();

Answer 1

如果出於某種原因要使用MySQL用戶變量， 則不需要多查詢支持 。 只要您打開會話（連接），它們（用戶變量）就會存在。 因此，您可以執行以下操作

$customer = 1;

try {
    $db = new PDO('mysql:host=localhost;dbname=dbname;charset=UTF8', 'user', 'password');
    $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

    $sql = "SET @available = (SELECT SUM(payments) FROM payments WHERE customer = ?) - 
                             (SELECT SUM(charges)  FROM charges  WHERE customer = ?)";
    $query = $db->prepare($sql);
    $query->execute(array($customer, $customer));

    $query = $db->prepare("SELECT @available");
    $query->execute();
    $result = $query->fetchAll(PDO::FETCH_ASSOC);
} catch (PDOException $e) {
    echo "Exeption: " .$e->getMessage();
    $result = false;
}
$query = null;
$db = null;

var_dump($result);

樣本輸出：

array(1) {
  [0]=>
  array(1) {
    ["@available"]=>
    string(6) "100.00"
  }
}

Answer 2

PDO似乎不提供對多查詢的任何形式化支持（但似乎確實有一些支持），但是mysqli可以。 不過，它們都沒有為准備好的多條語句提供支持。

您可以像這樣使用mysqli ：

$mysqli = new mysqli('servername', 'username', 'password', 'dbname');
$query = sprintf("Set @available = (SELECT SUM(payments) FROM payments WHERE customer = %1$s)" .
            " - (SELECT SUM(charges) FROM charges WHERE customer = %1$s);".
            " SELECT @available",
            $mysqli->real_escape_string($customer) );

 // Following code lifted from PHP Manual.
 // This code will read multiple results, if they're available.
 // Your query only returns one.

 /* execute multi query */
 if ($mysqli->multi_query($query)) {
    do {
        /* store first result set */
        if ($result = $mysqli->store_result()) {
            while ($row = $result->fetch_row()) {
                printf("%s\n", $row[0]);
            }
            $result->free();
        }
        /* print divider */
        if ($mysqli->more_results()) {
            printf("-----------------\n");
        }
    } while ($mysqli->next_result());
}

參考在這里