public function join($table, $criteria = null, $type = self::INNER_JOIN, $alias = null)
    {
        if (!$this->isJoinUnique($table, $alias)) {
            return $this;
        }

        if (is_string($criteria)) {
            $criteria = array($criteria);
        }

        $this->join[] = array('table' => $table,
            'criteria' => $criteria,
            'type' => $type,
            'alias' => $alias);

        return $this;
    }