mysql_num_rows() expects parameter 1 to be resource, string given (Opgelost)

function get_posts(){
    $sql = "SELECT
               `posts`.`post_id` AS `id`,
               `posts`.`post_title` AS `title`,
               LEFT(`posts`.`post_body`, 512) AS `preview`,
               `posts`.`post_user` AS `user`,
               DATE_FORMAT(`posts`.`post_date`,'%d/%m/%Y %H:%i:s') AS `date`,
               `comments`.`total_comments`,
               DATE_FORMAT (`comments`.`last_comment`, '%d/%m/%Y %H:%i:s') AS `last_comment`
          FROM `posts`
          LEFT JOIN (
              SELECT
                 `post_id`,
                 COUNT(`comment_id`) AS `total_comments`,
                 MAX(`comment_date`) AS `last_comment`
              FROM `comments`
              GROUP BY `post_id`
          ) AS `comments`
          ON `posts`.`post_id` = `comments`.`post_id`
          ORDER BY `posts`.`post_date` DESC";

          $num_rows = mysql_num_rows($sql);
          if ($num_rows == 0)
          echo "bla";

    $posts = mysql_query($sql);

    $rows = array();
    while(($row = mysql_fetch_assoc($posts)) !== false){
        $rows[] = array(
            'id'              => $row['id'],
            'title'           => $row['title'],
            'preview'         => $row['preview'],
            'user'            => $row['user'],
            'date'            => $row['date'],
            'total_comments'  => ($row['total_comments'] === null) ? 0 : $row['total_comments'],
            'last_comment'    => ($row['last_comment'] === null) ? 'never' : $row['last_comment']
        );
    }


    return $rows;
}

function get_posts(){
    $sql = "SELECT
               `posts`.`post_id` AS `id`,
               `posts`.`post_title` AS `title`,
               LEFT(`posts`.`post_body`, 512) AS `preview`,
               `posts`.`post_user` AS `user`,
               DATE_FORMAT(`posts`.`post_date`,'%d/%m/%Y %H:%i:s') AS `date`,
               `comments`.`total_comments`,
               DATE_FORMAT (`comments`.`last_comment`, '%d/%m/%Y %H:%i:s') AS `last_comment`
          FROM `posts`
          LEFT JOIN (
              SELECT
                 `post_id`,
                 COUNT(`comment_id`) AS `total_comments`,
                 MAX(`comment_date`) AS `last_comment`
              FROM `comments`
              GROUP BY `post_id`
          ) AS `comments`
          ON `posts`.`post_id` = `comments`.`post_id`
          ORDER BY `posts`.`post_date` DESC";
 
          $num_rows = mysql_num_rows($sql);
          if ($num_rows == 0)
          echo "bla";
 
    $posts = mysql_query($sql);
 
    $rows = array();
    while(($row = mysql_fetch_assoc($posts)) !== false){
        $rows[] = array(
            'id'              => $row['id'],
            'title'           => $row['title'],
            'preview'         => $row['preview'],
            'user'            => $row['user'],
            'date'            => $row['date'],
            'total_comments'  => ($row['total_comments'] === null) ? 0 : $row['total_comments'],
            'last_comment'    => ($row['last_comment'] === null) ? 'never' : $row['last_comment']
        );
    }
 
 
    return $rows;
}