如何避免此 PDO 异常：当其他未缓冲的查询处于活动状态时无法执行查询

Question

我想在页面中打印一个简单的表格，其中包含 3 列：建筑物名称、标签 和架构风格。如果我尝试检索建筑物名称和arch. styles 没有问题：

SELECT buildings.name, arch_styles.style_name
FROM buildings
INNER JOIN buildings_arch_styles
ON buildings.id = buildings_arch_styles.building_id
INNER JOIN arch_styles
ON arch_styles.id = buildings_arch_styles.arch_style_id
LIMIT 0, 10

我的问题始于为我刚刚编写的查询的每个构建检索前 5 个标签。

SELECT DISTINCT name
FROM tags
INNER JOIN buildings_tags
ON buildings_tags.tag_id = tags.id
AND buildings_tags.building_id = 123
LIMIT 0, 5

查询本身工作得很好，但不是我想使用它的地方：

<?php

// pdo connection allready active, i'm using mysql
$pdo_conn->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, true);

$sql = "SELECT buildings.name, buildings.id, arch_styles.style_name
        FROM buildings
        INNER JOIN buildings_arch_styles
        ON buildings.id = buildings_arch_styles.building_id
        INNER JOIN arch_styles
        ON arch_styles.id = buildings_arch_styles.arch_style_id
        LIMIT 0, 10";

$buildings_stmt = $pdo_conn->prepare ($sql);
$buildings_stmt->execute ();
$buildings = $buildings_stmt->fetchAll (PDO::FETCH_ASSOC);

$sql = "SELECT DISTINCT name
        FROM tags
        INNER JOIN buildings_tags
        ON buildings_tags.tag_id = tags.id
        AND buildings_tags.building_id = :building_id
        LIMIT 0, 5";
$tags_stmt = $pdo_conn->prepare ($sql);

$html = "<table>"; // i'll use it to print my table

foreach ($buildings as $building) {
    $name = $building["name"];
    $style = $building["style_name"];
    $id = $building["id"];

    $tags_stmt->bindParam (":building_id", $id, PDO::PARAM_INT);
    $tags_stmt->execute (); // the problem is HERE
    $tags = $tags_stmt->fetchAll (PDO::FETCH_ASSOC);

    $html .= "... $name ... $style";

    foreach ($tags as $current_tag) {
        $tag = $current_tag["name"];
        $html .= "... $tag ..."; // let's suppose this is an area of the table where I print the first 5 tags per building
    }

}
$html .= "...</table>";
print $html;

我在查询方面没有经验，所以我虽然像这样，但它抛出了错误：

PHP Fatal error:  Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active.  Consider using PDOStatement::fetchAll().  Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.

我能做些什么来避免这种情况？我应该更改所有内容并以不同的方式搜索来获取此类查询吗？

Answer 1

你说你发布了代码的简化版本。当您将其发布到此处时，您是否更改了其他内容？当您同时“打开”多个查询时，通常会导致此错误。例如，您调用 fetch()，但直到它耗尽才调用它，然后尝试检索第二个查询。

从上面的代码来看，这种情况不应该发生，因为您正在使用 fetchAll()。通常，解决这个问题的方法是调用 closeCursor() [文档]。您可以尝试在每个 fetchAll 之后调用它，看看是否有任何作用。

Answer 2

在循环中，您实际上再次获取第一个语句（注意

$buildings_stmt->fetchAll() 调用）：

$tags_stmt->execute ();
$tags = $buildings_stmt->fetchAll (PDO::FETCH_ASSOC);

您可能想要做的是获取 $tags_stmt陈述？

$tags_stmt->execute ();
$tags = $tags_stmt->fetchAll (PDO::FETCH_ASSOC);