SELECT * FROM pics p
WHERE p.id not IN (SELECT pics_id FROM votes WHERE user_id = XXX)

SELECT pics.id, pics.name 
FROM pics
LEFT JOIN 
  (
     SELECT pics_id
     FROM votes
     WHERE user_id = %d
  ) np
ON pics.id = np.pics_id
WHERE np.pics_id IS NULL  

SELECT t1.id,t1.name
FROM pics t1
LEFT JOIN votes t2 ON t2.`pics_id` = t1.`id`
WHERE t2.`pics_id` IS NULL

SELECT 
    `pics`.`id`, `pics`.`name` 
FROM
    `pics`
LEFT JOIN 
    `votes` 
ON 
    `votes`.`pics_id` = `pics`.`id` AND `votes`.`user_id` = 34 // Сюда подставить id пользователя
WHERE
    `votes`.`pics_id` IS NULL

SELECT 
    `pics`.`id`, `pics`.`name`
FROM  
    `pics` 
LEFT JOIN
    `votes` 
ON  
    `votes`.`pics_id` =  `pics`.`id` 
WHERE  
    `votes`.`user_id` <> 34 OR `votes`.`pics_id` IS NULL

			$this->db->select('`comparisons`.`file_name`, `comparisons`.`title`, COUNT(`votes`.`count`) as v');
			$this->db->from('comparisons');
			$this->db->join('votes', 'comparisons.id = votes.comparison_id', 'left'); 
			$this->db->group_by('votes.comparison_id');
			$this->db->order_by('v');
			$query = $this->db->get();
			$result = $query->result_array();