public function getSQL()
    {
        if ($this->sql !== null && $this->state === self::STATE_CLEAN) {
            return $this->sql;
        }

        switch ($this->type) {
            case self::INSERT:
                $sql = $this->getSQLForInsert();
                break;
            case self::DELETE:
                $sql = $this->getSQLForDelete();
                break;

            case self::UPDATE:
                $sql = $this->getSQLForUpdate();
                break;

            case self::SELECT:
            default:
                $sql = $this->getSQLForSelect();
                break;
        }

        $this->state = self::STATE_CLEAN;
        $this->sql   = $sql;

        return $sql;
    }