Is there a function to get the display name from the username?

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I'm looking for a function where you can pass the username and get the display name returned? I know I can do a query to the per_users table to get the person_id from the username and then link to the per_person_names_f table to get the display name, but I need to get the same data from 2 tables, getting both the created by and last updated by from each table. This would entail having 4 separate subqueries to get the values. It would be much simpler if I could just call a function, passing the username.