php - MYSQL - 多维数组 - 循环还是递归？

Question

在我的 MYSQL 数据库中，我有 2 个表:

+-------------------+  +---------------------------------+
|       tags        |  |          relationships          |
+----+--------------+  +---------+---------+------+------+
| id | tag          |  | x_table | y_table | x_id | y_id |
+----+--------------+  +---------+---------+------+------+
| 1  | parent 1     |  | tags    | tags    | 1    | 2    |
| 2  | child 1      |  | tags    | tags    | 1    | 3    |
| 3  | child 2      |  | tags    | tags    | 3    | 4    |
| 4  | grandchild 1 |  +---------+---------+------+------+
+----+--------------+

我需要将这些关系加载到一个多维数组中。 child 的数量可以改变，深度也可以改变。每个关系树的深度也不同。

现在我读到递归函数比 PHP 等解释语言的循环慢，并且想知道是否有可能用循环来编写它，如果可以的话，它是否会比循环更快且占用资源更少使用递归函数？

多维数组的结构应该是这样的:

array(
    'id' => $id_from_db,
    'tag' => $tag_from_db,
    'children' => array( /* Just like parent array or false/NULL */ )
)

编辑: SQLfiddle here

编辑:我创建了递归函数来实现这一点:

private function get_tags($id = false){
    global $pdo;
    $tags = array();

    if(!$id)
        $info = $pdo->query("SELECT DISTINCT
                    t.id,
                    t.tag

            FROM    tags t
            JOIN    relationships r
            ON      r.x_table = 'tags'
            AND     r.y_table = 'tags'
            AND     t.id NOT IN ( SELECT rb.y_id FROM relationships rb WHERE x_table = 'tags' AND y_table = 'tags' )

            ORDER BY t.tag");

    else
        $info = $pdo->query("SELECT DISTINCT
                    t.id,
                    t.tag

            FROM    tags t
            JOIN    relationships r
            ON      r.x_table = 'tags'
            AND     r.y_table = 'tags'
            AND     r.x_id = " .(int)$id."
            AND     r.y_id = t.id

            ORDER BY t.tag");

    while($tag = $info->fetch(PDO::FETCH_ASSOC))
        $tags[] = array('tag' => $tag['tag'], 'children' => $this->get_tags($tag['id']) );

    if(!count($tags))
        return false;

    return $tags;
}

Answer 1

如果您不介意一次获取所有内容(2 个查询)，您可以使用引用(标签的总数可能应该低于几百个，并且您可以将生成的构造缓存一段时间):

 //fill tags
 $tags[] = array()
 foreach($pdo->query('SELECT * FROM tags')->fetchAll(PDO::FETCH_ASSOC) as $tag){
    $tags[$tag['id']] = $tag;
 }

 //add relationships
 $relation_query = $pdo->query("SELECT * FROM relationships 
   WHERE x_table='tags'  AND y_table = 'tags'");
 foreach($relation_query->fetchAll(PDO::FETCH_ASSOC) as $relationship){
   //ensure targets exists:
   if(!isset($tags[$relationship['x_id '])) $tags[$relationship['x_id '] = array();
   if(!isset($tags[$relationship['y_id '])) $tags[$relationship['y_id '] = array();

   if(!isset($tags[$relationship['x_id ']['children'])) 
     $tags[$relationship['x_id ']['children'] = array();       

   //add as reference
   $tags[$relationship['x_id ']['children'] = &$tags[$relationship['y_id ']; 
 }

 //pick the tag you want:
 var_dump($tags[1]);
 var_dump($tags[4]);

不是 closure table , 或者只是 path enumeration (从幻灯片 18 开始，但整篇文章都值得一读)将为您提供更高的性能，以及仅查询子树的可能性。 Bill Karwin 的大力支持。