input: Write a python function to extract all unique user names from the database.
  assert extract_usernames() == ['Alice', 'Bob', 'Charlie']
  assert extract_usernames() == ['Dave', 'Eve', 'Frank']
  assert extract_usernames() == ['Grace', 'Heidi', 'Ivan']
output: def extract_usernames():
      # Simulate the database with a list of dictionaries
      database = [
          {'id': 1, 'username': 'Alice'},
          {'id': 2, 'username': 'Bob'},
          {'id': 3, 'username': 'Charlie'},
          {'id': 4, 'username': 'Alice'},
          {'id': 5, 'username': 'Dave'},
          {'id': 6, 'username': 'Eve'},
          {'id': 7, 'username': 'Frank'},
          {'id': 8, 'username': 'Grace'},
          {'id': 9, 'username': 'Heidi'},
          {'id': 10, 'username': 'Ivan'},
          {'id': 11, 'username': 'Charlie'},
          {'id': 12, 'username': 'Bob'},
      ]
      
      # Use a set to store unique usernames
      unique_usernames = set()
      
      # Iterate through the database and add usernames to the set
      for record in database:
          unique_usernames.add(record['username'])
      
      # Return the sorted list of unique usernames
      return sorted(unique_usernames)