[Solved] Delete not working properly as wanted

I have a column called code which stores a unique id when user previews a post.

If code is empty it should not remove the row. But for some reason deletes rows the code column is empty as well.

How can I make sure it only removes rows if code is present and has been there for more than 15min

PHP Code:

public function clear_unconfirmed_post() {
     $this->db->select('code');
     $this->db->from($this->db->dbprefix . 'post');
     $query = $this->db->get();

     if (!empty($query->row()->code)) {    
            // The where below is OK
            $this->db->where('date_created < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 15 MINUTE))', NULL, FALSE); 
            $this->db->delete($this->db->dbprefix . 'post');
     }
} 


I have tried

PHP Code:

public function clear_unconfirmed_post() {
        $this->db->where('code is NOT NULL', NULL, FALSE);
        $this->db->where('date_created < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 15 MINUTE))', NULL, FALSE);
        $this->db->delete($this->db->dbprefix . 'post');
} 


There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!