final public function sqlIsNull(string $field, bool $positive = true): string
    {
        if (! $positive) {
            trigger_error(
                __METHOD__ . ' with argument $positive = false is deprecated, use DBAL::sqlIsNotNull',
                E_USER_NOTICE
            );
            return $this->sqlIsNotNull($field);
        }
        return $field . ' IS NULL';
    }