Auto Actions - Progressive id only for user approved - assign a progressive number onUserApproval

Hello!
i'm trying to develop this actions:
onAfterUserApproval an unique, progressive number will be assigned to the user as reference number.
This id can't be the same of user_id because user_id is an autoincrement field and its value is assigned also for deleted and unapproved users.
I need to assign my reference number only to approved users...
Someone have an idea to solve my problem?

thanks at all!

P.

Create an integer field then use a Query action to select all the rows that have a value for the integer field and order it so the highest number is first then limit it to 1 result. Now do + 1 to the result and update the users row with the new incremented value. Query would look something like the below


Note the above is just an example. I have not tested it. Modify and test as needed.