左连接只取一行答案

【问题标题】：left join only fetches one row左连接只取一行
【发布时间】：2015-03-20 09:39:27
【问题描述】：

当我使用fetchAll() 时，此SQL 仅从表category 中获取一行，我知道至少有两行具有相同的信息。有什么想法吗？

Click here to see a more detailed explanation of what I thought the problem was

        $query = "        SELECT        
                                category.*,
                                GROUP_CONCAT('category_hierarchy.category_id' SEPARATOR ',') AS subcategories   

                FROM            category
                LEFT JOIN       category_hierarchy      ON      category.category_id = category_hierarchy.category_parent_id
                WHERE           category.type = '1'
                ORDER BY        category.sort_order ASC";


// Prepare.
    $stmt = $dbh->prepare($query);

// Execute.
    $stmt->execute();

// Fetch results.
    $categories = $stmt->fetchAll();

    $countedRows = count($categories);

    foreach($categories as $category) {
        $parent_arr = '';
        if(!empty($category['subcategories'])) {
            $parent_arr = array(display_children($category['subcategories']));
        } 

        $arr[] = array(
                'category_id'   => $category['category_id'],
                'title'         => $category['title'],
                'slug'          => $category['slug'],
                'url'           => $category['url'],
                'type'          => $category['type'],
                'sort_order'    => $category['sort_order'],
                'categories'    => $parent_arr
        );
    }

【问题讨论】：

标签： php mysql sql pdo

【解决方案1】：

您有一个聚合函数group_concat，并且没有group by 子句，它将始终返回一行，您可能需要在末尾添加group by。

SELECT      
category.*,
GROUP_CONCAT('category_hierarchy.category_id' SEPARATOR ',') AS subcategories   
FROM            category
LEFT JOIN       category_hierarchy      ON      category.category_id = category_hierarchy.category_parent_id
WHERE           category.type = '1'
group by         category.category_id
ORDER BY        category.sort_order ASC

【讨论】：