Is there any documentation on the While server action?

I want to insert a record which has a field for a unique slug. I want to check if it already exists in the database and, if it does, add a number to the end. Check it again and if it still exists, increment the number and try again. I do this until it doesn’t exist.

eg. myslug, myslug1, myslug2, myslug3, etc.

I’m thinking that the way to do this is with a While action but I can’t find any documentation on how to use it.

Should I put a query within the while loop? If so, what goes in the Expression?