As title, I think a work around is to pull data into a SP list form, then have the Nintex form to lookup the value there.
But I am wondering if it is possible to do it directly.
You have in Nintex Form an action to do that: "SQL Request" ('General' category).
You can read data directly in database.
the SQL Request mentioned before is available only if you are using Nintex Forms Enterprise, with that action, you'll get also a Web Request action, to bring data from a web service.
Thanks for the help, I am wondering if Nintex Forms Enterprise is available for SharePoint Online?
I'm afraid not.
Retrieving data ...