protected function buildUpdateSet(
        /*# string */ $prefix,
        array $settings
    )/*# : string */ {
        $result = [];
        foreach ($this->set_data[0] as $col => $val) {
            if (ClauseInterface::NO_VALUE === $val) {
                $result[] = $col;
            } else {
                $result[] = $this->quote($col, $settings) . ' = ' .
                    $this->processValue($val, $settings);
            }
        }
        return $this->joinClause($prefix, ',', $result, $settings);
    }