public function getPrimaryKey(array $pk, array $columns, $indent = 0)
    {
        if (empty($pk)) {
            return '';
        }

        // Composite primary keys
        if (2 <= count($pk)) {
            $compositePk = implode(', ', $pk);
            return $this->textIndent($indent) . "'PRIMARY KEY ($compositePk)'," . self::ENTER;
        }
        // Primary key not an auto-increment
        $flag = false;
        foreach ($columns as $column) {
            if ($column->autoIncrement) {
                $flag = true;
            }
        }
        if (false === $flag) {
            return $this->textIndent($indent) . sprintf("'PRIMARY KEY (%s)'," . self::ENTER, $pk[0]);
        }
        return '';
    }