是否可以在所需循环之外创建数据库调用？

Question

我正在尝试按使用数据库中的数据创建的平均值来显示和排序数组。我正在从数据库中检索三个变量并从这些值中创建一个平均值。然后将该值放入一个新数组中，以便与其余数据库数据一起排序。

我是否认为在循环中包含 SQL 查询不是一个好主意？ （性能问题？）

有没有其他可用的替代方案？我附上了以下代码：

    ^ database connection/query string to retrieve all data...
$result = $stmt_business_list->fetchAll(PDO::FETCH_ASSOC);

$items = array();
    foreach($result as $row){

    $single_business_id = $row['id'];
    $name = $row['name'];
    //Query to get ALL the service, value and quality ratings for certain business
    $test_query = "SELECT * FROM rating WHERE business_id = $single_business_id";
    $test_query_stmt = $dbh->prepare($test_query);
    $test_query_stmt->execute();
    $test_row = $test_query_stmt->fetchAll(PDO::FETCH_ASSOC);
    $total_value = $total_quality = $total_service = 0;

        foreach($test_row as $review)
        {
            $total_value += $review['value'];
            $total_quality += $review['quality'];
            $total_service += $review['service'];
        }

    $bayesian_value = (($set_site_average_review_count * $set_site_average_review_score) + $total_value) / ($set_site_average_review_count + $business_review_count);
    $bayesian_quality = (($set_site_average_review_count * $set_site_average_review_score) + $total_quality) / ($set_site_average_review_count + $business_review_count);
    $bayesian_service = (($set_site_average_review_count * $set_site_average_review_score) + $total_service) / ($set_site_average_review_count + $business_review_count);
    $average_bayesian_rating = ($bayesian_value + $bayesian_quality + $bayesian_service) / 3;
    $average_bayesian_rating = $average_bayesian_rating;

        array_push($items, array(
        "id"=>"$single_business_id",
        "name"=>"$name",
        "value"=>"$total_value",
        "quality"=>"$total_quality",
        "service"=>"$total_service",
        "average"=>"$average_bayesian_rating"));

    echo 
    'Name: '.$name.'<br>
    Value: '.$total_value.'<br>
    Quality: '.$total_quality.'<br>
    Service: '.$total_service.'<br>
    Average: '.$average_bayesian_rating.'<br><br>';
    }
}

该页面将由一个单独的分页脚本拆分，一次仅显示 6 个对象，但随着时间的推移，这可能会发生变化，因此我会尽可能多地关注性能。

Answer 1

SQL 聚合查询是针对这类事情进行的。

使用这个查询来总结结果

 SELECT b.name, b.id,
        SUM(value)   total_value,
        SUM(quality) total_quality,
        SUM(service) total_service,
        COUNT(*)     review_count,
        avg_reviews_per_biz
   FROM business b
   JOIN ratings r ON b.id = r.business_id
   JOIN (
          SELECT COUNT(DISTINCT business_id) / COUNT(*) avg_reviews_per_biz
            FROM ratings
        ) a ON 1=1                     
  GROUP BY b.name, b.id, avg_review_per_biz

这将为每个商家提供一行，显示总评分和评分数。此结果集将包含以下列

name            business name
  id            business id
  total_value   sum of value ratings for that business
  total_quality sum of quality ditto
  total_service sum of service ditto
  review_count  number of reviews for business "id"
  avg_reviews_per_biz   avg number of reviews per business 

对于查询的所有行，最后一列的值都相同。

然后，您可以一次循环遍历这些行一项业务来进行统计计算。

我无法从你的问题中看出你在哪里得到像 $set_site_average_review_count 这样的变量，所以我无法帮助这些计算。

你会发现SQL聚合查询确实很强大。