public function toSql()
    {
        if ($this->isEmpty()) {
            return '';
        }

        $params = $this->getParams();

        $sql = array('SET');
        $set = array();

        foreach ($params as $column => $value) {
            $set[] = implode(' = ', array(
                $column, $this->getBuilder()->getHelper()->toDbValue($value)
            ));
        }

        $sql[] = implode(', ', $set);

        return implode(' ', $sql);
    }