SELECT $wpdb->posts.* 
       FROM $wpdb->posts, $wpdb->postmeta
       WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id 
       AND ($wpdb->posts.post_title LIKE '%".$_POST['term']."%' OR ($wpdb->postmeta.meta_key = 'vnutrenniy_kod' AND $wpdb->postmeta.meta_value LIKE '%".$_POST['term']."%')) 
       AND $wpdb->posts.post_status = 'publish' 
       AND $wpdb->posts.post_type = 'post'
       GROUP BY $wpdb->posts.ID
       ORDER BY $wpdb->posts.post_date DESC
       LIMIT 5
    ";

SELECT * FROM demoshop_posts, demoshop_postmeta WHERE demoshop_posts.ID = demoshop_postmeta.post_id AND (demoshop_posts.post_title LIKE '%стакан%' OR (demoshop_postmeta.meta_key = 'vnutrenniy_kod' AND demoshop_postmeta.meta_value LIKE '%стакан%')) AND demoshop_posts.post_status = 'publish' AND demoshop_posts.post_type = 'post' GROUP BY demoshop_posts.ID ORDER BY demoshop_posts.post_date DESC LIMIT 5