我建议用wordpress的方式来代替sql查询：

$args = array(
\'numberposts\' => 3,
\'offset\' => 0,
\'category\' => put_your_category_id_here,
\'orderby\' => \'post_date\',
\'order\' => \'DESC\',
\'post_type\' => \'post\',
\'post_status\' => \'publish\',
\'suppress_filters\' => true );

$request = wp_get_recent_posts( $args, ARRAY_A );

Update : 以下是当您发布帖子时获取缩略图url的方法

    $post_thumbnail_id = get_post_thumbnail_id( $post_id );
    if($post_thumbnail_id){
      $imageurl = wp_get_attachment_url( $post_thumbnail_id );
    }

foreach( $recent_posts as $recent ){ 
    echo \'<a href="\' . get_permalink($recent["ID"]) . \'" rel="bookmark" title="\' . $recent["post_title"] . \'">
    <article>\';
      $post_thumbnail_id = get_post_thumbnail_id($recent["ID"]);
      if($post_thumbnail_id){
      $imageurl = wp_get_attachment_url( $post_thumbnail_id );
      echo \'<img src="\' . $imageurl . \'" alt="" class="latest-img" >\';
      }
      echo \'<img src="\' . get_template_directory_uri() .\'/img/g.png" class="g" width="667" height="266">
      <div class="latest-meta">
        <time class="hstack white">DATE</time>
        <h2 class="white gstack">\' . $recent["post_title"] . \'</h2>
      </div>
    </article>
    </a>\';
    }