There is no “new row” trigger node in n8n for SQL Server I am afraid. You could, however, implement your own polling logic, for example by adding a column like “processed_by_n8n” to your table.
Then have your workflow run in regular intervals (using the Interval or Cron node) and simply fetch all rows where processed_by_n8n is null, then update these rows and set processed_by_n8n to (for example) the current timestamp so they won’t be retrieved again next time your workflow runs.
Yes, I can see how such a trigger would be a very convenient thing to have. You might want to leave a vote on the respective feature request: Trigger workflow when new line added in SQL