非常简单的MySQL查询不起作用

SELECT d.* FROM as_downloads d LEFT JOIN as_categories c ON (d.download_category_id = c.category_id) WHERE d.download_category_id != -1 LIMIT 30

从 as_downloads 表,但不加入类别表。。

没有任何错误,我在PHPMyAdmin中测试过,结果是一样的,下面是使用的PHP代码

class Model_Downloads extends ModelType_PDO
{
    public function fetchDownloads($limit)
    {
        $p = Registry::get('Config')->Database->prefix;

        $query = "SELECT d.* FROM ".$p."downloads d LEFT JOIN ".$p."categories c ON d.download_category_id = c.category_id WHERE d.download_category_id != -1 LIMIT :limit";
        $this->query = $this->prepare($query);
        $this->query->bindValue(':limit',$limit,PDO::PARAM_INT);

        if($this->query->execute())
        {
            return $this->query->fetchAll(PDO::FETCH_CLASS);
        }
        return false;
    }
}