WordPress获取某个分类关联的标签

在使用WordPress做二次开发的过程中，遇到这样的一个需求，如何获取某个分类下属的标签？那什么是某个分类的下属标签呢，我们可以这么理解，如下图：

我在WordPress后台某篇文章的编辑页面，给这篇文章选择了分类：WordPress，接着同时选择了标签：php、主题制作，这时分类（WordPress）就与标签（php、主题制作）建立了关联，利用这种关联我们实现很多种需求，最典型的是细化WordPress文章分类功能，在子分类无法达到我们的需求时，可以使用标签来细化。我只需要提供WordPress分类的id，就可以得到它关联的标签：php、主题制作。

调用函数

我们可以先在主题目录的下的functions.php添加以下函数：

function ludou_get_category_tags($args) {
	global $wpdb;
	$tags = $wpdb->get_results
	("
		SELECT DISTINCT terms2.term_id as tag_id, terms2.name as tag_name
		FROM
			$wpdb->posts as p1
			LEFT JOIN $wpdb->term_relationships as r1 ON p1.ID = r1.object_ID
			LEFT JOIN $wpdb->term_taxonomy as t1 ON r1.term_taxonomy_id = t1.term_taxonomy_id
			LEFT JOIN $wpdb->terms as terms1 ON t1.term_id = terms1.term_id,

			$wpdb->posts as p2
			LEFT JOIN $wpdb->term_relationships as r2 ON p2.ID = r2.object_ID
			LEFT JOIN $wpdb->term_taxonomy as t2 ON r2.term_taxonomy_id = t2.term_taxonomy_id
			LEFT JOIN $wpdb->terms as terms2 ON t2.term_id = terms2.term_id
		WHERE
			t1.taxonomy = 'category' AND p1.post_status = 'publish' AND terms1.term_id IN (".$args['categories'].") AND
			t2.taxonomy = 'post_tag' AND p2.post_status = 'publish'
			AND p1.ID = p2.ID
		ORDER by tag_name
	");
	$count = 0;
	
	if($tags) {
	  foreach ($tags as $tag) {
	    $mytag[$count] = get_term_by('id', $tag->tag_id, 'post_tag');
	    $count++;
	  }
	}
	else {
	  $mytag = NULL;
	}
	
	return $mytag;
}

函数用法

然后在其他主题文件中，我们可以调用某个分类或某几个分类关联的标签：

// 12,13是分类ID，多个用半角逗号隔开
$args = array( 'categories' => '12,13');

// 调用上面定义的函数，来获取ID为12，13的分类的关联标签
$tags = ludou_get_category_tags($args);

// 输出我们获取到的关联标签，以列表形式打印
$content .= "<ul>";

if(!empty($tags)) {
  foreach ($tags as $tag) {
    $content .= "<li><a href="".get_tag_link($tag->term_id)."">".$tag->name."</a></li>";
  }
}

$content .= "</ul>";
echo $content;

好了，基本用法就这些，具体其他用法你可以自由地进行扩展。

获取某个标签关联的分类

反过来，我们可能会有这样的需求，既然可以获取某个分类的关联标签，那我能获取某个标签的关联分类吗？答案是可以的，将上面的代码稍微改一下就可以了：

function ludou_get_tag_categories($args) {
    global $wpdb;
    $categories = $wpdb->get_results
    ("
        SELECT DISTINCT terms1.term_id as cat_id
        FROM
            $wpdb->posts as p1
            LEFT JOIN $wpdb->term_relationships as r1 ON p1.ID = r1.object_ID
            LEFT JOIN $wpdb->term_taxonomy as t1 ON r1.term_taxonomy_id = t1.term_taxonomy_id
            LEFT JOIN $wpdb->terms as terms1 ON t1.term_id = terms1.term_id,
            $wpdb->posts as p2
            LEFT JOIN $wpdb->term_relationships as r2 ON p2.ID = r2.object_ID
            LEFT JOIN $wpdb->term_taxonomy as t2 ON r2.term_taxonomy_id = t2.term_taxonomy_id
            LEFT JOIN $wpdb->terms as terms2 ON t2.term_id = terms2.term_id
        WHERE
            t1.taxonomy = 'category' AND p1.post_status = 'publish' AND terms2.term_id IN (".$args['tags'].") AND
            t2.taxonomy = 'post_tag' AND p2.post_status = 'publish'
            AND p1.ID = p2.ID
        ORDER by cat_id
    ");
    $count = 0;
   
    if($categories) {
      foreach ($categories as $category) {
        $mycategory[$count] = get_term_by('id', $category->cat_id, 'category');
        $count++;
      }
    }
    else {
      $mycategory = NULL;
    }
   
    return $mycategory;
}

调用范例：

// 12,13是标签ID，多个用半角逗号隔开
$args = array( 'tags' => '12,13');

// 调用上面定义的函数，来获取ID为12，13的标签的关联分类
$categories = ludou_get_tag_categories($args);

// 输出我们获取到的关联分类，以列表形式打印
$content .= "<ul>";
if(!empty($categories)) {
  foreach ($categories as $category) {
    $content .= "<li><a href="".get_category_link( $category->term_id )."">".$category->name."</a></li>";
  }
}
$content .= "</ul>";
echo $content;

参考文档
http://wordpress.org/support/topic/get-tags-specific-to-category